The Open/Closed Principle

Entities should be open for extension and closed for modification.

The second of the 'SOLID' principles. This principle states that entities (which could be classes, modules, functions and so on) should be able to have their behaviour extended, but that their existing behaviour should not be able to be modified.

As a hypothetical example, imagine a module which is able to turn a Markdown document into HTML. Now imagine there is a new syntax added to the Markdown specification, which adds support for mathematical equations. The module should be open to extension to implement the new mathematics syntax. However, existing syntax implementations (like paragraphs, bullets, etc) should be closed for modification. They already work, we don't want people to change them.

This principle has particular relevance for object-oriented programming, where we may design objects to be easily extended, but would avoid designing objects which can have their existing behaviour changed in unexpected ways.