#1
I always here people on hear talking about diatonic scaled and stuff and I look around on with google to see what it means, so can anyone tell me what diatonic means.
Skip the username, call me Billy
#2
Diatonic means sticking within a seven note scale.

For example if I were to do something "Diatonic" in reference to the key of C major then I would be using the notes from C major (C D E F G A B) only.

At least that's how I think of it.
Si
#3
A tonic is the root note of a scale, the 1st degree. Diatonic uses the greek/latin/italian(i don't know) prefix dia- meaning two. That is because each scale has two tonics, the first and eighth scale degrees. For example, C major scale would be C D E F G A B C. The last C is an octave above the first C, so it's still the tonic note. Since there are two tonics in a scale, it's diatonic.