How to Use “Define” in Arduino?

This is an interrupt service routine that is used for timing routines. Sometimes it is difficult to follow time intervals in code, because you may need to create a delay or delay in action. As you know, Arduino uses a relay circuit where we need some time gap between two actions on the system, and Define() helps establish this delay. It's not used to define variables (like you might use in other programming languages), because there are already abstractions for that in Arduino--int, float, String, etc.

The most important thing is to remember what define() does: it defines a constant that can be used within your sketch, such as a pin number, delay time, you name it.