Margulis lemma

Etymology
Named after.

Proper noun

 * 1)  A result about discrete subgroups of isometries of a non-positively curved Riemannian manifold, stating roughly that, within a fixed radius (the Margulis constant), the structure of the orbits of such a group cannot be too complicated. More precisely, within this radius around a point all points in its orbit are in fact in the orbit of a nilpotent subgroup (in fact a bounded finite number of such).