I've been wondering this for a while now. Xander started saying "dada" when he was a mere 7 months old. It made his father light up but I didn't count it as a word because it seemed to be simple babbling.
When, at 9 months old he started saying "mama", I completely discounted it as well, nice as it made me feel. However, when he was scared by a vacuum or got water in his eyes or nose in the bath, "MAMA!" was what he would yell. After this, I wasn't sure.
When we are holding him and he drops a toy - or something similar happens - we say "Uh oh!". I did not even pay that much attention to how often we say it until he started saying it back. Again, I discounted it as simple mimicking. Then I noticed that whenever he drops a toy or someone else does, he looks up at them and says "uh oh". ...clearly he has associated it with accidents. Is this his first real word? So I started wondering - when are the sounds he makes a word?
Was "dada" his first word despite him probably only babbling it?
Was that simple title "mama" his first word?
Was his first real word "uh oh"? ...or does that sound even count as a word at all?
When he decided that the cats were called "ah..T" (soft ah, slight pause, hard T) at 8 months, was that a word?
It's a tricky question!!!