What is a 9 mm bullet?

A bullet with a diameter of approximately 9mm. Not all 9mm projectiles are exactly 9mm in diameter - the 9x19 Parabellum/Luger has a diameter of 9mm, while the 9x18 Makarov has a diameter of 9.27mm, the 9x23 Steyr has a diameter of 9.03mm, etc.