What is "half-diminished seventh chord"

n. (context music English) a chord with the bass note and the minor third, diminished fifth, and minor seventh above it

In music theory, the half-diminished seventh chord—also known as a half-diminished chord or a minor seventh flat five (m75)—is formed by a root note, a minor third, a diminished fifth, and a minor seventh. Its consecutive intervals are minor 3rd, minor 3rd, major 3rd.

It can be created by playing the tonic, flattened third, flattened fifth, and flattened seventh degrees of any major scale (1, 3, 5 and 7)—for example, C half-diminished is (C E G B). Alternatively, it can be derived simply by raising (by a semitone) the root note of the major seventh chord.

In diatonic harmony, the half-diminished chord occurs naturally on the 7th scale degree of any major scale (for example, B in C major). By the same virtue, it also occurs on the second degree of any natural minor scale (e.g., D in C minor). It occurs as a leading-tone seventh chord in major and can be represented by the integer notation {0, 3, 6, 10}. It has been described as a "considerable instability".