(set theory) a set that is a subset of the set, but not equal to it.

(mathematics) A set which serves as the codomain of a characteristic function. / (category theory, uncountable) The subobject classifier restricted to the category Set.

plural of subset classifier

(commutative algebra) A subset of a ring which contains the ring’s unity and which is multiplicatively closed, i.e., it is closed with respect to the ring’s multiplicative operation.