Abbreviated as NCT for learning purposes, a non-chord tone is a tone (either
diatonic or
chromatic) that is not a member of the chord.
The tone might spend it's entire played duration as a non-chord tone; however, if the harmony that surrounds it changes before the actual tone itself does, the NCT might change to a
chord tone before it ends.
NCT's are often times
passing tones or
neighboring tones.