What does define mean?

Answer 1

"define" is an action-based word, meaning "to assign something a definition".
So if you define a function, you are creating it, and assigning it a task.
If you define a variable, you are creating it, and assigning it a value.
Simple as that.

Answer 2

define is a PHP function. Is used to define (or declare) constants in your code.

Answer 3

The define function is used to create a constant

Answer 4

define a constant. a value that can't be changed