Axiom:Axiom of Unions/Also known as

Axiom of Unions: Also known as
The axiom of unions is in fact most frequently found with the name axiom of union.

However, in some treatments of axiomatic set theory and class theory, for example Morse-Kelley set theory this name is used to mean something different.

Hence specifically uses the plural form axiom of unions for this, and reserves the singular form axiom of union for that.

Some sources refer to the axiom of unions as the Axiom of the Sum Set.