Why is a CALLBACK function named CALLBACK?

[Eric.Erpelding]

I think I understand the operation of GLBasic's CALLBACK functions.
A CALLBACK function can be overloaded by a function with the same name, essentially replacing the CALLBACK function with the non-CALLBACK function over the whole GLBasic project.

Can anyone explain why the CALLBACK function is designated using the word CALLBACK?

The Wiki article on CALLBACK states:

In computer programming, a callback is a piece of executable code that is passed as an argument to other code, which is expected to call back (execute) the argument at some convenient time.

So in GLBasic what code is CALLing BACK?