Group center

In abstract algebra the center of a group G denoted by Z(G) is the set of elements which commute with every element of G, meaning that for any element x of a group G, and an element y of Z(G), xy = yx. The center is equal to the intersection of the centralizers of the group elements. 
The notation Z(G) comes from the fact that in German the word for center is "Zentrum".