PEP 21, or Python Enhancement Proposal 21, is a document that outlines a specific improvement or feature for the Python programming language. It serves as a formal way for developers to propose changes, enhancements, or new features to Python, ensuring that the community can review and discuss these ideas. PEPs are an essential part of Python's development process, helping to maintain the language's quality and consistency. The proposal process involves several stages, including drafting, community feedback, and final approval by the Python Steering Council. PEP 21 specifically focuses on the Python 3 series, aiming to enhance its functionality and usability for developers.