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 commutative property
The additive property.
Commutative property (Abelian property) of addition.
It is the commutative property of addition.
It is the commutative property of addition.
Commutativity.