A minor major seventh is the chord you get when you take a
minor triad and add to it the seventh note of the
major scale. They're notated
Cm-maj7 or in
jazz as
C-Δ7 or
CmΔ7.
For example, an A minor triad is A C E. The seventh note of the A major scale is G#, so the Am-maj7 chord consists of A C E G#.
Rather a harsh-sounding chord, owing to the augmented triad and semitone dissonance it contains.
Compare with the major seventh, minor seventh and dominant seventh chords.