In music theory, a minor seventh is an interval that has ten half steps 
The minor seventh requires that:
- The interval must be a seventh interval (seven note names between the first and the last).
- The interval must have ten half steps.
What does a minor seventh look like?
Here is an example of a melodic minor seventh (two music notes in a melody) and a harmonic minor seventh (in a chord):
How to recognize a minor seventh?
Rule of music theory: All intervals in a major scale starting with the tonic (degree I) are either major or perfect, and only unison, octave, fourth and fifth are perfect (the others are major).
Example with the interval C / B♭:
Let's take the C major scale to have C as the tonic:
From the rule stated above, the interval C / B♮ is a major seventh, so C / B♭ is a minor seventh.
Inversion of the minor seventh
The inversion of the minor seventh is the major second.
Here is an example of a major second:
Musical examples of minor seventh
Somewhere (West Side Story) by Bernstein
The Somewhere theme from Bernstein's West Side Story begins with a minor seventh:
Interval identification game
You will find this interval in my Intervals identification game:
