Big O Notation

This is a notation which is commonly used to express the time or space complexity of an operation. In other words, how quickly the time/space required for the operation grows.

It does not necessarily indicate how long (or how much space) the operation will actually take. For example, an operation that always takes 100 seconds is O(1), and an operation that starts at 0.0001 seconds and grows by 0.0001 seconds each time is O(n). But it would be better to use the latter unless you're going to need it more than a million times.