The way I accepted it after years of confusion, and without still understand the word: A monad is any type which supports 2 specific operations. One maps an type into it's monadic form, and the other allows to apply functions to monadic types and returns the monadic type again. The latter is sometimes called bind or flatmap.

For Future types, the first thing could be called MakeReadyFuture(type), the second one Future.then(result => functionWhichReturnsANewFuture).