In mathematics, and specifically in group theory, a non-abelian group, sometimes called a non-commutative group, is a group ( G, ∗ ) in which there exists at least one pair of elements a and b of G, such that a ∗ b ≠ b ∗ a. This class of groups contrasts with the abelian groups. (In an abelian group, all pairs of group elements commute).
Non-abelian groups are pervasive in mathematics and physics. One of the simplest examples of a non-abelian group is the dihedral group of order 6. It is the smallest finite non-abelian group. A common example from physics is the rotation group SO(3) in three dimensions (rotating something 90 degrees along one axis and then 90 degrees along a different axis is not the same as doing them the other way round).