Diatonic and Chromatic
If you were discussing a passage in C major, a diatonic note would be one drawn from the scale of C major. The opposite of diatonic is chromatic - referring to notes not from the diatonic scale that the passage in question is based on. Diatonic is obviously a relative term because a Db, for example, would be diatonic in a passage based on a Db major scale, but chromatic in a passage based on a C major scale.
In the example below, the main notes are those of the diatonic C major scale while the smaller notes in brackets are examples of chromatic alterations of these notes: