The four types of triad in the key of C.
Augmented is to be raised by a half-step (semitone). The term is applied both to intervals and chords in which one of the notes has been so raised. Thus, an augmented triad is a triad consisting of a root, a major third, and an augmented fifth – in other words, two major thirds, stacked one on top of the other. For example C augmented (shown in the illustration here) consists of the notes C, E, and G♯.
An augmented seventh is an augmented triad plus a dominant (minor) seventh interval above the root. The name is a bit misleading because it's the fifth not the seventh which is augmented. Other names for this chord are the augmented minor seventh, the seventh augmented fifth, and, in jazz, the seventh sharp five. C augmented seventh (abbreviated to Caug7), for example, consists of the notes C, E, G♯, and B♭. An augmented major seventh, also called a major seventh sharp five, is an augmented triad with an added major seventh.
Augmented chords are fairly rare in rock music but common in jazz, in which the fourth, fifth, and ninth are often found augmented.
The opposite of augmented is diminished.