there's a word for these words ...

What is the word for words that have two opposite meanings? Like cleave or sanction(s)? Whatever it is, I thought of another expression that doesn't quite fit, but I think deserves a mention in the same category: give it up. It can mean "to quit," or also to clap frenetically, as in, "give it up for your Cincinnati Reds!"

And then there's words (I know there's a different word for them) that just have two utterly and completely different meanings that aren't opposites -- like peer (n., person of a similar age or rank vs. v., to look intently) or lap (n., what happens to your legs when you sit down vs. v., to drink like a dog).

Don't you just love letters??