The name has its origin in type theory (you also have product types). It's not the best name I guess, but not the worst either: If we have type A with n terms a

The name has its origin in type theory (you also have product types). It's not the best name I guess, but not the worst either: If we have type A with n terms and type B with m terms, then the sum type of A and B has n + m terms. There are much more confusing things in type theory, like the ambiguity that the word 'type' has (I still don't grok the terminology, to be honest).