First Words?

What constitutes a baby's first word? I mean the girls are already using the typical baby sounds of 'mmma' and 'bbbba' and even 'da da da da.' So aside from those I believe that the girls have said their first words (after 15 months of age).

Natalie
Yesterday I was playing with the girls. Fiona and Jadon went to out to Costco. I was playing with a little plush lamb that when you press the stomach, it says a bed time prayer that ends off with "amen." Well I did it initially and I noticed Natalie saying "ama." I press the lamb again and as soon as it finished Natalie said it again. I repeated the process a few more times after that but couldn't get another response from her again. I tried a few minutes later and there she repeated "ama" again. So I'm going to officially record that Natalie's first word is "amen."

Vanessa
It's a little harder to distinguish what Vanessa's first word is. Yesterday I got her to say "ball." She was playing with a plush football and I was telling her that it was a "football, ball, ball." And she ended up saying "ba ba." She repeats it twice everytime she says it. So I would definitely say that is a word that she is saying but I think that perhaps that she has been saying "bye" long before that. When she sees one of us leaving out the garage she'll yell out "Bla! Bla!" and wave her hand.

It'll will be interesting to see when they both talk. Hopefully it won't get as annoying as Jadon can be sometimes. All those questions (most of them repeated over and over and over).