Definition:

Module: The word module can refer to one of two things. In the general sense, a module is a complete software subsystem that can stand on its own. Modules can be loaded into programs to extend their functionality and to be re-used in other programs.

In Ruby, a module is a collection of methods that can be used to extend a class. Modules are "included" in classes to extend their functionality.

Though Ruby modules are similar to modules in other languages, they can only be used with classes. Ruby uses the keyword require to import modules into a program.