# Function

A specific purpose of an entity; its characteristic action.

A relationship between two sets of values, where the second value (the "output") is uniquely determined (according to some clearly specified "rule") by the first value (the "input").

### Bound

A curve that encloses the major peaks of the envelope of a function.

### Envelope

A curve that is tangent to the peaks of the function.

**See also: **Constant Functions, Domain of the Function, Invertible, Linear Function, One to One, Piecewise Function, Rate of Change.

**Subjects: ** General