空间群的不对称单元是空间的一个简单连接的最小闭合部分，通过应用空间群的所有对称运算，整个空间都被填充。这意味着：
镜像平面必须形成不对称单元的边界平面；
旋转轴必须形成不对称单元的边界边缘；
反演中心必须形成不对称单元的顶点，或者位于边界平面或边界边缘的中点。
这些限制不适用于螺旋轴和滑动平面。
术语“不对称单元”并不意味着该区域具有非对称形状。在数学中，它被称为基本域或基本域。
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 asymmetric unit;
rotation axes must form boundary edges of the asymmetric unit;
inversion centres must either form vertices of the asymmetric 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 'asymmetric unit' does not mean that this region has an asymmetric shape. In mathematics it is called fundamental region or fundamental domain.
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