direct sum

Short definition from Wiktionary

Noun

1. 1
   coproduct in some categories, like abelian groups, topological spaces or modules
2. 2
   A linear sum in which the intersection of the summands has dimension zero. Equivalently, a linear sum of two subspaces, any vector of which can be expressed uniquely as a sum of two vectors: one vector belonging to the first subspace, and the other vector belonging to the second subspace.
3. 3
   A block diagonal matrix interpreted as having its non-zero blocks (which are square matrices) as summands.