How would you define a tritone substitution?

Tritone substitution is a popular reharmonization method, where the bass note of a Dom7 V chord (can be the main dominant or any auxiliary dominant before any chord) gets shifted a tritone away.

The 3-b7 of the chord that create the release tension of the Dominant chord remain the same (eg. G7 with B-F becomes Db with F-Cb).

When altering a chord, the altered notes all remain functional as well (eg. G7#9b13 with Bb, Eb extensions becomes a Db13). Y

ou can truly “substitute” the chord but also very popular is to use both in conjunction, meaning to add the tritone sub additionally to add anticipation.

Tritone substitution works because the two chords share two pitches: namely, the third and seventh (reversed).

In a G7 chord, the third is B and the seventh is F; whereas, in its tritone substitution, D♭7, the third is F and the seventh is C♭ (enharmonically B♮).

Notice that the interval between the third and seventh of a dominant seventh chord is itself a tritone.