Can someone explain me, what is the meaning of the term "Compact Fundamental Domain" in the following theorem? 


"Every discrete group of isometries acting on the n-dimensional euelidean space R^n with compact fundamental domain contains n linearly independent translations" ? 


Thanks in advance ! 



Bieberbach's Theorem
A fundamental domain X is a subset such that:


1) If U is the interior, each G-orbit intersects U at most once.


2) If C is the closure, then each G-orbit intersects C at least once.


