Commutative Operation

Formal Definition
A binary operation $*$ on a set $S$ is commutative if, for all $a,b\in S$, $a*b=b*a$

Informal Definition
If binary operation yields the same output for any given pair of inputs in its domain, regardless what the order of the inputs is, the operation is commutative.

Example(s)
Addition is a commutative binary operation. This is illustrated by fact that, $1+2=2+1=3$.
Multiplication is a commutative binary operation. This is illustrated by fact that $5\times 4 = 4\times 5=20$

Non-example(s)
Subtraction is an example of a binary operation that is not commutative. For example, $2-1$ is not equal to $1-2$.