n (uncountable|mathematics) A system for computation using letters or other symbols to represent numbers, with rules for manipulating these symbols.
n (countable|algebra) An algebraic structure consisting of a module of a commutative ring along with an additional binary operation that is bilinear.
n (countable|set theory|analysis) A collection of subsets of a given set, such that this collection contains the empty set, and the collection is closed under unions and complements (and thereby also under intersections and differences).