What is a module in Python?

A module in Python is fundamentally a file that contains Python code. This code can define functions, classes, or variables, and can also include runnable code. By organizing code into modules, developers can encapsulate and reuse code effectively across different parts of a program or even across different projects. This modular approach promotes better code organization and maintainability.

Modules allow for reusability; once a module is created, it can be imported and utilized in other Python programs without needing to rewrite the code. This is a key feature of Python's design that contributes to its efficiency as a programming language.

The other options describe concepts that do not accurately represent what a module is. For instance, while certain external libraries may include modules, not all modules are external libraries, nor do they require installation in the same way that larger libraries might. Additionally, neither a shorthand for writing code nor a type of data structure correctly describes a module, as a module specifically refers to a file containing Python code, rather than a format or data collection method. Therefore, the answer that defines a module as a file containing Python code that can define functions, classes, or variables accurately encompasses its purpose and functionality in the Python language.