Assuming that there is no significance in the first X being upper case and the second in lower case, it is an IDENTITY.
Commutative
A turn-around fact
symmetric property
Commutative Property of Addition
The commutative property of addition
the commutative property of addition
The additive property.
The commutative property
It is the commutative property of addition.
Commutative property (Abelian property) of addition.
It is the commutative property of addition.
Commutativity.