Non-chord tone
A note that does not belong to the sounding chord, e.g. passing tone or suspension.
What it means
A non-chord tone (German harmoniefremd, “alien to the harmony”) is a note that does not belong to the chord currently sounding. It falls outside the harmony and therefore acts like a small dissonance that usually moves on to a chord tone after a short time. Well-known types are passing tone, neighbour note, suspension, anticipation, and escape tone.
Musical use
Non-chord tones enliven a melody and link the chord tones together. A passing tone fills stepwise the space between two chord tones; a suspension delays the entry of a chord tone and thus creates tension that resolves. Their effect depends on their position in the bar: on a strong beat they sound weightier than on a weak one.
A non-chord tone should be distinguished from a chord tone, the main notes belonging to the chord, and from modulation, in which the harmonic reference point changes. A note can be non-chord even when it lies within the key, as long as it is not part of the current chord.
Performance practice
When playing, recognise which notes are non-chord tones and give them a slight weight with a pull toward resolution: stress the suspension and let the resolution follow more softly. This makes the tension of the passage audible.