A tone which has
harmonic relevancy to what is going on in the
harmony at the time of occurance.
If a C major chord is comprised of the notes "C" "E" and "G", any of those notes can be a chord tone. This gets stickier when doing more complex
musical analyses when the lowest note is not necessarily the
bass note of the chord you are dealing with.
Example: You are reading a
score and you see F in the bass voice, with A and C above it amongst the several
simultaneous tones occuring on that beat. You assume it is a F major chord and move on. You would be wrong however, because not only is F major not diatonic in the
key signature the piece you are looking at is in, but C is the seventh degree in the
Dmaj7 chord you are actually looking at.
Assuming you are looking at a
diatonic, tonal piece of music.
See? I wasn't kidding. Sticky.