Chirality: The geometric property of a rigid object (or spatial arrangement of points or atoms) of being non-superposable on its mirror image
If the object is superposable on its mirror image the object is described as being achiral.

Advanced notion, related to symmetry: such an object has no symmetry elements of the second kind (a mirror plane σ, a centre of inversion i, a rotation-reflection axis S_2n).

Reference: IUPAC GoldBook