Kazhdan-Margulis theorem

Etymology
Proven in the 1960s by David Kazhdan and Grigory Margulis.

Proper noun

 * 1)  A statement asserting that a discrete subgroup in semisimple Lie groups cannot be too dense in the group. More precisely, in any such Lie group there is a uniform neighborhood of the identity element such that every lattice in the group has a conjugate whose intersection with this neighborhood contains only the identity.