Might-have-beens are the lives, wins, or choices that never happened, but that you still turn over in your mind — the job you did not take, the words you did not say, the player who almost made it. We almost always say it in the plural, and it is a little sad or wistful. Though actually people use it when they want to stop themselves as much as when they want to dream: “No point in thinking about might-have-beens.” It is the ordinary phrase might have been, frozen into a name for a thing, then given a plural -s. Hyphens hold the pieces together so you can see it is one idea. You hear it in conversation and in newspapers; you would not put it in a science paper.