(physics) A theorem, proven by Emmy Noether in 1915 and published in 1918, stating that any differentiable symmetry of the action of a physical system has a corresponding conservation law.

(politics) A theorem stating that no voting system can be perfectly fair in all circumstances

(economics) A theorem stating that, if trade in an externality is possible and there are sufficiently low transaction costs, bargaining will lead to a Pareto efficient outcome regardless of the initial allocation of property.

(group theory) The theorem stating that every group G is isomorphic to a subgroup of the symmetric group acting on G.

The theorem that any graph labelling (with colours) of a sufficiently large complete graph contains monochromatic cliques.

(mathematics) A theorem stating that any non-singular algebraic curve defined by algebraic number coefficients represents a compact Riemann surface which is a ramified covering of the Riemann sphere, ramified at three points only.

(computing theory) A theorem stating that all nontrivial semantic properties of programs are undecidable.

(graph theory) A characterization of the connectivity in finite undirected graphs in terms of the minimum number of disjoint paths that can be found between any pair of vertices.

(quantum mechanics) A method of reducing high-order derivatives to a combinatorics problem, used extensively in quantum field theory to reduce arbitrary products of creation and annihilation operators to sums of products of pairs of these operators.

(chiefly mathematics) A theorem (or, in non-mathematical fields, a commonly accepted hypothesis) considered to be of central importance to a specified field.