What keyword is used to define a class in Python?

In Python, the keyword used to define a class is "class." This keyword serves as the foundational element for creating a user-defined class, which allows you to encapsulate data and functionality in an object-oriented manner. When you use the "class" keyword followed by the class name, you initiate the declaration of a new class that can contain methods and attributes, providing a blueprint for creating instances of that class.

Understanding this structure is essential for implementing object-oriented programming principles in Python, such as inheritance, encapsulation, and polymorphism. The other options do not represent any syntactical elements in Python for class definition and would not be recognized by the interpreter, which reinforces that "class" is indeed the correct choice.