From Online Dictionary of Crystallography
Unité asymétrique (Fr). Unità asimmetrica (It). 非対称単位 (Ja).
An asymmetric unit of a space group is a simply connected smallest closed part of space from which, by application of all symmetry operations of the space group, the whole space is filled. This implies that:
- mirror planes must form boundary planes of the asymetric unit;
- rotation axes must form boundary edges of the asymetric unit;
- inversion centres must either form vertices of the asymetric unit or be located at the midpoints of boundary planes or boundary edges.
These restrictions do not hold for screw axes and glide planes.
The term "aysmmetric unit" does not mean that this region has an asymmetric shape. In mathematics it is called fundamental region or fundamental domain.