A dichotomy is a division into two entirely different and often contrasting domains, interests or activities (1). Examples of true dichotomies are black or white. Tall or short.
A false dichotomy is an argument giving a false illusion of there being only two choices whereas in reality there can be at least one other or even many possibilities (2). The argument is set up in such a way as the first choice is eliminated due to it being seen as a terrible choice, and the only other alternative is the second choice.