Originally Posted By: The Pook
Similar concept

I'd like to hear your reasoning behind deciding that the two things are similar.
Retronym = At one time, all guitars were not electric. Some one invents an electric guitar. For a while, you have guitars and "electric guitars". At a later time, guitar comes to mean "electric guitar", so you have guitars and "acoustic guitars" instead.
Backronym = The Pook means The Person Of Obscure Knowledge. (unless I guessed that right ;-) ).