The term center is used in various contexts in abstract algebra to denote the set of all those elements that commute with all other elements. More specifically:
The center of a ringR is the subset of R consisting of all those elements x of R such that xr = rx for all r in R. The center is a commutative subring of R, so R is an associative algebra over its center.
The center of an algebraA consists of all those elements x of A such that xa = ax for all a in A.
The center of a Lie algebraL consists of all those elements x in L such that [x,a] = 0 for all a in L. This is an ideal of the Lie algebra L.
The center of a groupG consists of all those elements x in G such that xg = gx for all g in G. This is a normal subgroup of G. See center of a group for more information.
