1. the use or purpose of something.

2. in biology, an activity of an organ or an organism that contributes to the organism’s fitness, such as the secretion of a sex hormone by a gonad to prepare for reproduction or the defensive behavior of a female with young toward an intruder.

3. (symbol: f) a mathematical procedure that relates or transforms one number, quantity, or entity to another according to a defined rule. For example, if y = 2x + 1, y is said to be a function of x. This is often written y = f(x). Here y is the dependent variable and x is the independent variable.