Recommendations from the Ad Hoc Committee on SETI Nomenclature
Jason T. Wright , Sofia Sheikh1, Iván Almár , Kathryn Denning , Steven Dick and Jill
1 2 3 4
The Ad Hoc Committee on SETI Nomenclature was convened at the suggestion of
Frank Drake after the Decoding Alien Intelligence Workshop at the SETI Institute in
March 2018. The purpose of the committee was to recommend standardized definitions
for terms, especially those that are used inconsistently in the literature and the scientific
community. The committee sought to recommend definitions and terms that are a
compromise among several desirable but occasionally inconsistent properties for such
1. Consistency with the historical literature and common use in the field
2. Consistency with the present literature and common use in the field
3. Precision of meaning
4. Consistency with the natural (i.e. everyday, non-jargon) meanings of terms
5. Compatibility with non-English terms and definitions
The definitions below are restricted to technical, SETI contexts, where they may have
jargon senses different from their everyday senses. In many cases we include terms
only to deprecate them (in the sense of “to withdraw official support for or discourage
the use of…in favor of a newer or better alternative”, Merriam-Webster sense 4).
This is a consensus document that the committee members all endorse; however, in
many cases the individual members have (or have expressed in the past) more
nuanced opinions on these terms that are not fully reflected here, for instance Almár
(2008, Acta Astronautica, 68, 351), Denning (2008, NASA-SP-2009-4802 Ch. 3
pp.63–124), and Wright (2018, arXiv:1803.06972).
1 Penn State University Center for Exoplanets and Habitable Worlds and Department of Astronomy &
Astrophysics, PA, USA
2 Hungarian Academy of Sciences, Budapest, Hungary
3 Associate Professor, York University Department of Anthropology, Toronto, ON, Canada
4 Former NASA Chief Historian and Former Baruch S. Blumberg NASA/Library of Congress Chair in
5 Chair Emerita for SETI Research, SETI Institute, Mountain View, CA, USA
A subfield of astrobiology focused on searching for signs of non-human technology
or technological life beyond Earth. The theory and practice of searching for
extraterrestrial technology or technosignatures.
Of that subfield
(the) Search for Extraterrestrial Intelligence
is an acronym and so should be pronounced (sɛ’ti in American English),
not spelled out.
Especially at NASA, SETI
has sometimes been more narrowly defined to refer
specifically to radio SETI, or to a particular program at NASA. The term generally has
broader scope in the SETI literature and community.
should not be used as shorthand for the SETI Institute, which is an independent
entity and should be referred to by its full name to avoid confusion.
When referring to that which SETI seeks, shorthand for life or technology not originating
recently on Earth.
The terminology is complicated by the possibility that Earth life originated
elsewhere or has significantly spread beyond our planet (i.e. panspermia hypothesis),
and the fact that human technology is present throughout our Solar System.
Although the term in general has broader meaning than this, even in astronomy, its use
in the SETI context to refer to non-terrestrial life is so entrenched that we endorse this
jargon sense and will not attempt to refine it to be more precise. Alternatives, such as
have similar difficulties and are not yet in wide use in the SETI
So by this definition, life on another planet with a common origin to Earth life but which
diverged billions of years ago would be extraterrestrial
, but Earth life accidentally
brought to Mars on a human-built lander would not. Similarly, humanity’s solar system
probes are not extraterrestrial
by this jargon definition.
In the acronyms SETI and ETI, the quality of being able to deliberately engineer
technology which might be detectable using astronomical observation techniques.
: Definitions of intelligence are slippery and much broader than technological
so we recommend against the term’s use as a synonym for technological
in general. Its
use in the acronym SETI is sufficiently entrenched that we recommend against a more
precise rebranding of the field.
Extraterrestrial Technological Entities or
Extraterrestrial Technological Species or
Extraterrestrial Intelligence or
The entities that created the technosignatures SETI seeks to find.
: To be preferred to alien race
or alien civilization
because of human connotations
in those terms. ETI
is not ideal because it contains intelligence
but is acceptable
because it directly references the term SETI
We note that some rarely seen alternatives such as sophont
(literally “wise one”) and
(those responsible for technosignatures) are useful because they lack
connotations about the entities’ nature except their ability to create technology.
However, none has yet gained sufficient traction in the community to earn our
endorsement. Extraterrestrial society
may be preferable to species
because it does not
assume a narrow biological species definition, but is not in wide use in the literature.
1. n. Deprecated term for extraterrestrial species
2. adj. Term to be used with care, meaning of or pertaining to extraterrestrial species or
: As a noun, to be avoided for many reasons, including its associations with
unscientific portrayals of extraterrestrial life in popular culture and its legal meaning in
relation to immigration.
We do not recommend the use of alien
as an adjective in general, but acknowledge it is
a common and useful term, so instead recommend that the term be used with care and
consideration of the issues we note with the noun. That is, alien species
is to be
preferred to alien
, but extraterrestrial species
In a SETI context, e.g. (extraterrestrial
or alien civilization
) usually synonymous with
. Use with care.
The term civilization
has imprecise popular meanings, but also particular
scholarly meanings in relation to human history that are not generally what is meant by
the term in a SETI context. Because of its ambiguity and anthropocentrism, the term is a
suboptimal synonym for technological species,
but it is nonetheless widely used in the
is a good alternative but not yet in common use.
Advanced (technology, species, etc.)
Deprecated term describing a technology or species that manipulates energy or matter
in a manner or extent which surpasses humanity’s capabilities.
: This term unhelpfully echoes deprecated theories of human history which rank
human societies from “primitive” to “advanced” based on ill-defined and ethnocentric
measures. While acknowledging the cumulative nature of human science and
technology, we recommend instead simply specifying the scale or nature of the
technology referenced (for instance, using the Kardashev scale, physical size, etc., as
The physical manifestations of deliberate engineering. That which produces a
Any sign of technology that is not also a biosignature (i.e. that is not also a sign or
byproduct of metabolism in common with non-technological species).
: By analogy with biosignature
refers to signs or signals, which, if
observed, would allow us to infer the existence of technological life elsewhere in the
universe; this tracks the natural meaning of the word and the intent of its original
coinage (see Tarter (2007) Highlights of Astronomy 14, 14). In some cases, the line
between biosignatures and technosignatures may be unclear, and technology need not
be of biological origin, as in the case of “postbiological evolution.”
Not being a product of deliberate engineering; antonym of artificial
Being a product of deliberate engineering; antonym of natural
: These terms are important because they are part of the boundaries between
SETI and the rest of astrobiology. Unfortunately, they are slippery, and are not even
well defined for observable phenomena on Earth. For instance, a beaver’s dam or the
tunnels under an anthill are somewhat ambiguous by these definitions. It is also unclear
to what degree we will recognize extraterrestrial artifice as such if and when we find it.
Any “we are here” sign or signal deliberately engineered by a technological species to
be noticed, recognized, and understood by another technological species as evidence
or proof of the first technological species’ presence.
: This is useful because searches for such signals are amenable to game
theoretical approaches (i.e. magic frequencies, Schelling points, etc.). It also matches
the natural meaning of the term. Sometimes the term is used to refer only to
content-free signals (as in the original meaning of “beacon”, i.e. a lighthouse or signal
fire), but we recommend other terms for this sense (see dial tone).
Dial Tone or
A content-free beacon, i.e. one that communicates only
the existence of technological
: This concept may be useful in discussions of the social consequences of
detection, because human reactions to an extraterrestrial signal without content may be
quite different from reactions to the contents of an actual extraterrestrial message.
These terms for the concept are perhaps too colloquial and parochial (in particular dial
may become obscure in the near future) but we include them because they convey
the sense of the concept well by analogy to their common meanings, and because they
are already in use in the SETI community.
Colonize (and their cognates)
The (hypothetical) spread of a species across space, in particular throughout a
planetary system, among stars, and throughout a galaxy.
is the more common term in the SETI literature, but some avoid (or
embrace) its use because of its connotations of the global colonial exploits of European
1. n. A subfield of SETI dedicated to the search for physical manifestations of
technology, exclusive of communicative transmissions.
: This sense is useful because it captures many approaches to SETI which differ
in strategy or philosophy from searches for deliberate transmissions meant for
communicative purposes. Note that searches for communicative transmissions are
often also sensitive to non-communicative transmissions; for instance, radio searches
may be sensitive to extraterrestrial radar signals or signs of propulsion.
includes searches for probes in the Solar System (which are sometimes
called Solar System SETI or Probe SETI), and searches beyond our Solar System for
atmospheric technosignatures, signs of propulsion, and waste heat from industry.
often appears in the SETI literature in a narrower sense, to refer to
searches for artifacts in the Solar System, exclusively. We recommend the broader
sense because it is useful, closer to the natural meaning of the term, and already in use
in the SETI literature.
In the literature, artifact SETI
is sometimes called Dysonian SETI
, but that term more
naturally refers to searches for specific technosignatures suggested by Freeman Dyson,
especially Dyson Spheres.
There is no common term in the literature for searches for communicative
transmissions, specifically. Communication SETI
appears in the literature but is easily
confused with CETI. Both it and transmission SETI
have ambiguous natural meanings
and are easily confused with METI.
1. n. Deprecated term for the search for physical artifacts, such as probes in the Solar
2. acronym (the) Search for Extraterrestrial Artifacts
: This term was originally used to distinguish searches for Solar System artifacts
from searches for communicative transmissions. Because we endorse a broader
definition of SETI, we recommend that such searches be considered a subset of SETI,
not a distinct activity, and so we deprecate SETA, which is not in wide use today,
1. n. The activity of humans eliciting contact with technological species via the
transmission from Earth of beacons or other invitations for contact (including the
Arecibo messages and Pioneer plaques).
2. acronym Messaging Extraterrestrial Intelligence
The word is an acronym and so should be pronounced (mɛ’ti in American
English), not spelled out.
METI is a controversial activity. Some consider it to be logically continuous with SETI,
and others consider it to be a distinct activity. To some it also includes replies to future
hypothetical incoming transmissions, and theoretical work on how to communicate with
ETI, but others consider these to be distinct from METI.
should not be used as shorthand for METI International, which is an independent
entity and should be referred to by its full name to avoid confusion.
Deprecated term for METI
: Avoid this term because of syntactic ambiguity (e.g. “Dr. X is PI of three active
SETI programs”, “Dr. Y is an active SETI researcher”)
(game theory) An equilibrium in a non-communicative cooperative game such as a
mutual search; i.e. a mutually obvious game strategy. The most obvious example is the
consideration of “magic frequencies” for radio beacons, such as the 21 cm line in radio
: The identification of magic frequencies in SETI as a quintessential example of
this important game theoretical concept was established by Thomas Schelling himself in
1960. The term is synonymous with the jargon term focal point
in game theory, but
is to be preferred in astronomical contexts to avoid confusion with the
term in optics.
has priority over and is to be preferred to terms in the literature that have
not caught on such as mutual strategy of search
, or convergent strategy
of mutual search
1. n. Deprecated term encompassing: 1) SETI, 2) the theory of encoding and decoding
interstellar messages, and 3) METI.
2. acronym Communication with Extraterrestrial Intelligence
The term is rarely seen any more, and so we deprecate it in favor of SETI
The Kardashev Scale measures the energy supply of an entire technological society in
one of three integer types, as described by Kardashev in 1964 in Soviet Astronomy
217. Other senses of the term, including extensions of the scale, should be explicitly
described or cited.
There is no canonical formulation of the Fermi Paradox (see Gray (2015) Astrobiology
15, 195) but its earliest invocations refer to the (supposed) inconsistency between
hypothetical timescales for settlement of the Galaxy and the absence of extraterrestrial
spacecraft or probes on Earth. When used, the intended meaning should be given
Proper citation to most common form of the equation, which describes the number of
potentially detectable sources of radio transmissions in the Galaxy, is
Drake, F (1965) The Radio Search for Intelligent Extraterrestrial Life, in Current Aspects
, ed. G Mamikunian and M. H. Briggs, 323-45. Oxford University Press.
When using the term in other contexts, define precisely what N
is intended to represent.
A scale from 1–10 developed by astronomers to express their estimates of the
importance of a report of detection or contact with an extraterrestrial species, computed
as the product of a claim’s credibility and level of consequences. The scale was
adopted by the International Academy of Astronautics SETI Permanent Committee and
is currently at version 1.2, available at http://www.setileague.org/iaaseti/riocalc.htm.
An updated scale, “Rio 2.0” was recently proposed by Forgan et al. (International
Journal of Astrobiology, accepted
) but has not been adopted by the IAA and so
currently has no official status.