One of the most popular jazz chord substitution is the tritone substitution sometimes referred to as dominant chord substitution.

It consists in replacing a dom7 chord by another dominant 7th chord whose root is a tritone away from this initial chord. Example with G7 (V), which can be replaced by Db7 which is a tritone (three whole-steps) away from G7

Thus giving two chords that have two notes in common. The 7th of G7 (F) is the third of Db7 and the third of G7 (B) is the seventh of Db7. The inversion of the 3rds and the 7ths between the original dominant chord (V) and the substituted dominant chord (bII7) is the main feature of the tritone substitution.