en.wikipedia.org/wiki/Function_(mathematics)

The above definition of "a function from X to Y" is ... With one definition saying the
function defined by f(x) = x<sup>2</sup> ...

www.mathsisfun.com/definitions/function.html

A function is a special relationship where each input has a single output. It is
often written as "f(x)" where x is the input value. Example: f(x) = x/2 ("f of x is x ...

mathinsight.org/definition/function

A technical definition of a function is: a relation from a set of inputs to a set of
possible outputs where each input is related to exactly one output. This means
that ...

www.khanacademy.org/math/algebra/algebra-functions/intro-to-functions/v/what-is-a-function

Functions assign a single unique output for each of their inputs. ... Math by
subject .... This was kind of an interesting way to define a function, a way to kind
of ...

www.mathopenref.com/function.html

Functions - how they transform one value into another in a defined way.

www.thefreedictionary.com/mathematical function

Noun, 1. mathematical function - (mathematics) a mathematical relation such that
each element of a given set (the domain of the function) is associated with an ...

www.icoachmath.com/math_dictionary/function.html

Function is a relation in which each element of the domain....complete
information about the function, definition of an function, examples of an function,
step by ...

www.regentsprep.org/regents/math/algtrig/atp5/lfunction.htm

Relations and Functions. ... Definition of a Relation ... Function: A function is a set
of ordered pairs in which each x-element has only ONE y-element associated ...

www.regentsprep.org/Regents/math/algtrig/ATP8/inverselesson.htm

A function and its inverse function can be described as the "DO" and the "UNDO"
functions. A function takes a starting value, performs some operation on this ...

www.reference.com/math/function-rule-mathematics-55fcb260037f06a1

A function is a relationship in math between two variables, often x and y, and for
every value of x there is exactly ... What is the definition of "plane trigonometry"?