here's a quote from our own Anu Garg:

I call them fence-sitters. They sit on the fences, ready to say one thing or its opposite, depending on which side they appear. I'm not talking about politicians. These are words, known by many names: autoantonym, antagonym, contranym, enantiodromic, amphibolous, Janus word, and so on.

actually, that middlish one should be enantiodrome (and I suppose the next is amphibole), to maintain noun forms.