formal sum

Noun

 * 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.
 * 2)  An element of a free abelian group or vector space regarded as a sum of basis elements.
 * 3)  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.