Magma is a see also of monoid.

In context|mathematics|lang=en terms the difference between magma and monoid

is that magma is (mathematics) a basic algebraic structure consisting of a set equipped with a single binary operation while monoid is (mathematics) a set which is closed under an associative binary operation, and which contains an element which is an identity for the operation.

As nouns the difference between magma and monoid

is that magma is (geology) the molten matter within the earth, the source of the material of lava flows, dikes of eruptive rocks, etc while monoid is (mathematics) a set which is closed under an associative binary operation, and which contains an element which is an identity for the operation.