A Facet on the very bottom of a diamond, less used in Round Brilliant Cuts in the past couple decades. It is best to have a small or medium culet, or none. Most round brilliant cut diamonds today do NOT have a culet.
A large culet will make it look like there is a hole in the bottom of the stone due to leakage of light. Absence of a culet makes the point of the diamond more easily damaged or chipped, but most settings accommodate this issue and make it so the tip is well protected.