(mathematics) In additive combinatorics, a central result that indicates the approximate structure of sets whose sumset is small. It roughly states that if |A+A|/|A| is small, then A can be contained in a small generalized arithmetic progression.

(geometry) A statement in Euclidean geometry that deals with the construction of a particular parallelogram (the Varignon parallelogram) from an arbitrary quadrilateral.

(logic, mathematics) One of two theorems in mathematical logic that demonstrates the inherent limitations of every formal axiomatic system containing basic arithmetic.

(mathematics) A theorem stating that there is no nonvanishing continuous tangent vector field on the sphere; colloquially, that "you can't comb a hairy ball flat without creating a cowlick".

(mathematics) the theorem that there is at least one prime number between n and 2n for every n>1, i.e.:

(mathematics, computing theory) A theorem stating that, when applying reduction rules to terms in the lambda calculus, the ordering in which the reductions are chosen makes no difference to the eventual result.

(mathematical logic) A theorem stating that a certain combinatorial principle in Ramsey theory, namely the strengthened finite Ramsey theorem, is true, but not provable in Peano arithmetic.

(graph theory) A theorem stating that the undirected graphs, partially ordered by the graph-minor relationship, form a well-quasi-ordering.

(game theory) A theorem that describes a necessary and sufficient condition for the non-emptiness of the core of a cooperative game in characteristic function form. The game's core is non-empty if and only if the game is balanced.

(linear algebra) A theorem about linear transformations (or the matrices that represent them) stating that the rank plus the nullity equals the dimension of the entire vector space (which is the linear transformation’s domain).