Gender neutral pronoun.
In everyday use, I often use the pronoun "they" to refer to a unknown person if I do not know their sex. As in:
Bob: Who was it that emailed you about the company cutting back?
Sally: I don't know. They didn't say.
I remember using "they" this way in a paper for my college English 101 class, and my teacher marked off for it, saying that it is incorrect. So is it? It's definitely much easier to say than "he/she"...
Perhaps it's considered correct (or at least accepted) in speech but not in written form?