Wiktionary
n. (context music English) a chord with the bass note, the minor third, perfect fifth, and the minor seventh above it
Wikipedia
In music, a minor seventh chord is any nondominant seventh chord where the " third" note is a minor third above the root.
Most typically, minor seventh chord refers to where the "seventh" note is a minor seventh above the root (a fifth above the third note). This is more precisely known as a minor/minor seventh chord, and it can be represented as either as m7 or -7, or in integer notation, {0, 3, 7, 10}. In a natural minor scale, this chord is on the tonic, subdominant, and dominant degrees. In a harmonic minor scale, this chord is on the subdominant degrees. In an ascending melodic minor scale, this chord is on the supertonic degree. In a major scale, this chord is on the second (supertonic seventh), third ( mediant) or sixth ( submediant) degrees. For instance the ii in the ii-V-I turnaround.
Example of tonic minor seventh chords include LaBelle's " Lady Marmalade", Roberta Flack's " Killing Me Softly with His Song", The Doobie Brothers' " Long Train Runnin'", Chic's " Le Freak", and the Eagles' " One Of These Nights".
When the seventh note is a major seventh above the root, it is called a minor/major seventh chord. Its harmonic function is similar to that of a "normal" minor seventh, as is the minor seven flat five or half-diminished chord – but in each case, the altered tone (seventh or fifth, respectively) creates a different feeling which is exploited in modulations and to use leading-tones.