(mathematics) A related function that, given the output of the original function returns the input that produced that output.

(linear algebra) Of a matrix A, another matrix B such that A multiplied by B and B multiplied by A both equal the identity matrix.

Either concept, the inverse of the other, of a conceptual pair.

(mathematics) Proportional to the reciprocal of the independent variable.

(mathematics) For a given function f, another function, denoted f⁻¹, that reverses the mapping action of f; (formally) given a function f:X→Y, a function g:Y→X such that, ∀x∈X,f(x)=y⟹g(y)=x.

(category theory) For a given morphism f : X → Y, its right inverse (if it has one) is a morphism s : Y → X such that f∘s= mbox id_Y.

(algebra) A subset of the Cartesian product of all the members of an inverse system, such that a member M of the subset is a sort of “cross section” of the inverse system (as fiber bundle) induced by the morphisms of it. (If i<j in the indexing poset then f_ij:A_j→A_i in the inverse system and if a_i∈A_i, a_j∈A_j are components of M then f_ij(a_j)=a_i).

(algebra) A set of algebraic structures (which are part of a concrete category; e.g., groups) and a set of morphisms between them (e.g., group homomorphisms) which all together form a small category which is the image of a contravariant functor whose domain is a directed poset.

the inverse element with respect to multiplication; the reciprocal

arccosine