What is a metaclass in Python?

A metaclass is a 'class of a class' that defines how other classes are created, initialized, and behave. In Python, type is the default metaclass. Custom metaclasses allow you to modify class creation, automatically add methods or attributes, validate class definitions, or implement patterns as singletons. They are an advanced feature summarized by Tim Peters as: 'Meta classes are deeper magic that 99% of users should worry about.'