Function means the purpose something serves or the job it does. Every tool, body part, or piece of equipment has a function. The function of your heart is to pump blood. The function of a hammer is to drive nails into wood. When something works the way it's supposed to, we say it's functioning properly.

When describing what something does, you might say “This button functions as an on-off switch” or “The old barn now functions as a storage shed.” If your bike chain breaks, your bike can't function because it can't do its job of moving you forward.

In math, a function is a special relationship where one number depends on another in a predictable way. If you're paid $10 per hour, your earnings are a function of how many hours you work: work 3 hours, earn $30. Every input (hours worked) produces exactly one output (money earned). Think of a function like a reliable machine: put something in, get something specific out, every single time.

As a noun, function can also mean a formal event, like a school function or a community function.