Noun
dominant seventh chord (plural dominant seventh chords)
(music) a chord with the bass note and the major third, perfect fifth, and the minor seventh above it
A tritone (the interval of the fourth step to the seventh step of the major scale, i.e., F to B) sounds very dissonant alone, but less so within the context of a dominant seventh chord (G7 or D 7 in that example). Source: Internet
In a jazz or blues song, the tonic chord may be a dominant seventh chord. Source: Internet