direct sum
Mathematics
noun
A set regarded formally as the sum of sets X 1,…,X n, consisting of ordered n-tuples (x 1,…,x n), where x i ∈ X i, and distinguished from a direct product only in the case of an infinite number of sets, the direct sum consisting only of sequences containing a finite number of non-zero elements.
Origin
Early 20th century; earliest use found in Proceedings of the London Mathematical Society.