Diatonic

A scale is diatonic when it is the major scale or one of its variations - the natural minor and the seven modes all count. The word comes from Greek for "across the octave", and it describes the shape: seven notes spread evenly across the twelve-note octave, with no gap bigger than a whole step and the two half-steps kept well apart.

Although there are seven diatonic scales (the modes, major and minor among them), there is really only one diatonic structure. The seven are the same seven-note pattern started from different points - begin the major scale's shape on its second note and you get a different scale, but built from the identical set of notes. That shared structure is why they all belong to one key signature.

Diatonic versus chromatic

The opposite of diatonic is chromatic: a note that does not belong to the current key. When a chord or melody stays inside the seven notes of the key it is diatonic; reach outside them and you have borrowed a chromatic note. This is the line our Chords in a Key page draws when it tells you a chord is "diatonic" or "borrowed", and it is why a key can give every note a single, definite spelling.