Ain't ain't in my dictionary (Oxford English, which is British English so it may be in American English dictionaries). It's not a word in the same sense that can't, shan't, won't, wasn't, weren't and so on aren't words; they're contractions of words.
Still, the definition of words as 'words which are in the dictionary' only applies to prescriptivists, which I am not.
I personally do count can't/shan't/won't/etc. as words.