What is the difference between a plan and a strategy?

Is the difference related to the topic concerned (plan = internal problems, strategy = external problems): as Ansoff (1965) mentions “The study of strategy is the part of the study of decision making which is "primarily concerned with external, rather than internal, problems of the firm”

Is it related to a temporal distance (plan = short or medium term, strategy = long term)

Or is it related to the distance towards the subject (plan = concrete, strategy = abstract): "Planning is a formalized procedure to produce an articulated result, in the form of an integrated system of decisions. (Mintzberg, 1993)

In many articles, plan and strategy seem to be used interchangeably. So is there a difference and which one?