A first word?

How do you know when it’s actually a word?  Nicky’s said “ma-ma” and “da-da” before and he says “da-da” all the time now, but he doesn’t know what they mean; he’s just babbling.  Chuck told me that on Saturday while I worked, Nicky said “head” after Chuck said it.  He said it again a few times last night, but I think it’s just because he can but he doesn’t understand it yet.  It’s really cute; he says it like “head-ah”.