Should be used with the (group-theory) tag. A group $(G,*)$ is said to be abelian if $a*b=b*a$ for all $a,b\in G.$
An abelian or commutative group is a group $(G,*)$ in which all elements commute: $$\forall a,b\in G, a*b=b*a$$. Usually the product is denoted by $+$, and the identity of the group by $0$.
Examples include the integers $\mathbb{Z}$ under addition, as well as the rationals $\mathbb{Q}$ under addition. In fact, every cyclic group is also an abelian group. Non-examples include $S_3$, the symmetry group on three elements, as well as $SO(3)$, the rotations in three dimensions.