addition, fundamental operation of arithmetic, denoted by +. In counting, a + b represents the number of items in the union of two collections having no common members (disjoint sets), having respectively a and b members. In geometry a + b might, for example, represent the area of the union of two disjoint regions of areas a and b, respectively. In arithmetic addition follows the associative law, the commutative law, and, in combination with multiplication, the distributive law. Addition is also defined for other types of mathematical objects, for example, vectors and tensors. See also subtraction.