(computing) A function key whose function can be assigned by the user or by a program.

(physics) A function which is a solution of Schrödinger's wave equation; the psi function

(computing) A program or subroutine, provided by the user rather than being built into the system, that may return a value and may take input parameters.

(mathematics) Any function in which each point in the domain maps to just one point in the range

(mathematics) Any function whose value may be computed using a Turing machine.

(probability theory) Any function whose integral over a set gives the probability that a random variable has a value in that set

(medicine) Any of several diagnostic procedures that test the blood clotting function of platelets

plural of higher-order function

(computer science) A function that takes one or more functions as an input, and returns a function as a result.

(number theory, analytic number theory, uncountable) The function ζ defined by the Dirichlet series 𝜁(s)=∑ₙ₌₁ ᪲1/(nˢ)=1/(1ˢ)+1/(2ˢ)+1/(3ˢ)+1/(4ˢ)+⋯, which is summable for points s in the complex half-plane with real part > 1; the analytic continuation of said function, being a holomorphic function defined on the complex numbers with pole at 1.