formal sum

Short definition from Wiktionary

Noun

1. 1
   An expression which looks like a sum but where the symbol + has a purely symbolic role, in the sense that the summands cannot be meaningfully combined: An element of a free abelian group or vector space regarded as a sum of basis elements.
2. 2
   An expression which looks like a sum but where the symbol + has a purely symbolic role, in the sense that the summands cannot be meaningfully combined: A sum of an infinite sequence of numbers considered as an indeterminate object, in the sense of being subject to certain algebraic manipulations but not necessarily convergent.