# Definition $Z_2$, the cyclic [[group]] of [[group order|order]] 2, is a group with only two [[group element|elements]], e.g. 0 and 1, with the group operation being addition modulo 2. Note that *all* two-element groups are [[Group Isomorphism|isomorphic]] to $\mathbb{Z}_2.$ # In Physics It appears in the [[Z2 topological invariant]]