In Latex, what does X_n \asymp Y_n mean ?

I've tried to find the answer from various places:
https://groups.google.com/forum/#!topic/sci.math/2DIwkFqATXE

http://mathoverflow.net/questions/51761/does-fxgx-imply-fx-asymp-gx

But I somehow I find these explanations a bit confusing

In my case, I am talking about 2 sequences of numbers X_n and Y_n, so in latex, what does X_n \asymp Y_n mean ?

I really appreciate any help !

P/S Now does it make sense to have X_n \asymp Y_n, given that X_n is non-increasing and Y_n is non-decreasing ?