Minor 7 chords, the more serious relative of the regular 7 chord…
Here are ways you will see this notated as symbols:
Xm7 is most common most clear.
Take a dominant 7 chord, then change 3 to b3. There is close to zero problem with spelling, and frankly I don’t know why. There are variant spellings of the regular 7 chord, but generally not for this one. It may be that the spelling is more stable because it is not normally used as a modulating chord.