(mathematics) Any metric space whose elements are functions

(algebraic geometry) Informally, any function expressible using (only) the operations of addition, subtraction, multiplication, division and raising to a rational power; more precisely, any continuous function definable as the root of some polynomial equation.

(mathematics) Either of a pair of functions that have the same domain and that the values of one are equal to or greater than the other for all points in that domain.

(mathematics) Any function in which the interchange of two independent variables changes the sign of the dependent variable.

Any mathematical relationship between an activity and its cost.

(mathematics) Any function of a complex variable that is holomorphic throughout the complex plane

(mathematics) Any function that is a solution of the Bessel equation

(computing) Any character used to specify a function of a piece of software or hardware (such as carriage return of a printer)

(mathematics) Any function of a real variable whose value decreases (or is constant) as the variable increases.

(physics) The relationship between the energy of an electron and its ability to excite an atom to a particular excited state