Secondary Dominant refers to the use of dominant chords of any diatonic chord. Diatonic chord in simple and practical terms mean ‘in the key of’. So the diatonic chords in the key of C are:
C Dm Em F G Am Bdim
If we use the dominant chord for any of these chords, which happens to be a non-diatonic chord in the key of C, we can label that as a secondary dominant. To know what is meant by the dominant chord, please read this (
