First Word ~ how do you define it?

Everyone seems to know their babies first word, but to me it seems to be more of a gradual progression...  Riley is very noisy, she babbles away and really seems like she is trying to tell us something but she's not saying words.  But then again when my Mum was saying down to the dog she seemed to be trying to copy but it wasn't very clear and like wise she will try to copy ta when we give and take things...  she says dada all the time but it doesn't seem pointed at Rich particularly clearly so when is 'first word' defind? Anyway maybe it will become clear but at the moment she seems to be learning new sounds and trying to copy us which to me seems like she's going in the right direction :)