What is the runtime referred to when using define in PHP?

When using define in PHP, the runtime refers to the point at which the constant is defined and its value is set. It is important to define constants at the beginning of the script before any other code that may rely on them. This ensures that the constant is available throughout the runtime of the script.

define('MY_CONSTANT', 'some value');

// Rest of the PHP code that uses MY_CONSTANT