Non-diatonic note
A note or chord that lies outside the diatonic scale of a key, usually produced by chromatic alteration.
What it means
A non-diatonic note is one that does not belong to the seven scale degrees of the prevailing major or minor key. Such notes typically arise through chromatic alteration of a diatonic note — for instance, an F sharp appearing in a passage in C major instead of F natural.
The opposite term is diatonic: a note that is an unaltered member of the scale. Whether a note counts as non-diatonic always depends on the key in question; the same pitch can be diatonic in one key and non-diatonic in another.
Musical use
Non-diatonic notes enrich the harmony without the music actually leaving its home key. They appear above all in chromatic passing and neighbour notes, in altered chords such as secondary dominants or the augmented triad, and in chromatic melodic lines.
This should be distinguished from an actual modulation, where the key itself changes so that formerly foreign notes become diatonic in the new key. Non-diatonic notes, by contrast, remain exceptions within an established key and usually resolve quickly back to diatonic notes.
Performance practice
When sight-reading, watch for accidentals that do not belong to the key signature — they almost always signal a non-diatonic note. Play or sing the passage on its own first, so you notice the characteristic color or friction it creates before fitting it back into its musical context.