(mathematics) Any function whose value may be directly calculated from the independent variable.

(computing) Any of a set of subroutines that perform standard mathematical functions included in a programming language; either included in a program at compilation time, or called when a program is executed

(mathematics) Any function whose value changes sign if the independent variable changes sign i.e. f(-x) = -f(x)

(mathematics) Either of a pair of functions whose inner product is zero

(mathematics) a meromorphic function

(statistics) Any function used to obtain a set of samples from a given population

(chemistry) The relationship between the acidity of a solution and the concentration of its solute.

(mathematics) Any function in which an independent variable is in the form of an exponent; they are the inverse functions of logarithms

(mathematics) Any function of a complex variable which is periodic in two directions.

(mathematical analysis, algebraic geometry) A function defined by a (multivariable) implicit equation when one of the variables is regarded as the value of the function, especially where said equation is such that the value is not directly calculable from the other variables.