All 4 Uses

function
in
The Three-Body Problem
(Edited)

This explanation posits that external observation leads to the collapse of the quantum wave function.
p. 17.6 *

function = mathematical expression that describe a relationship between variables
After careful examination of both eyes, the doctor told him they were functioning normally with no signs of any disease.
p. 86.8 *

functioning = working
You're the key to the correct functioning of the gate.
p. 212.9

functioning = operation (to work properly)
The Roche limit is usually expressed as a function of the densities of the bodies and the equatorial radius of the larger body.
p. 238.9

function = mathematical expression that describe a relationship between variables

Definitions:

(1) (function as in: Its main function is to...)
purpose, job, or natural activity
(2) (function as in: can't function well without sleep)
work or operate -- sometimes to work or operate normally or in a particular way
(3) (function as in: I'm attending a function tonight)
a social event or ceremony -- especially one attended for work or out of a sense of duty
(4) (function as in: oxygen is a function of altitude)
saying one results from another; or saying that the amount of one thing is dependent upon the value of another.

If x is a function of y, then x results from y; or the amount of x can be calculated based on the amount of y.
(5) (function as in: Amazon's new search function)
computer code that performs a specific task
(6) (function as in: a mathematical function)
a formula or mathematical expression that describes a relationship between variables
(7) (meaning too rare to warrant focus)
For more specialized meanings in math and chemistry, see a comprehensive dictionary.