To create a reusable visual element, you often define a custom widget as a class extending ______.

To create a reusable visual element in Flutter, you often define a custom widget as a class extending 'StatelessWidget.' 'StatelessWidget' is suitable for elements that do not depend on mutable state. It helps in creating efficient, immutable UI components that do not change over time, making them ideal for static visual elements. Understanding when to use 'StatelessWidget' is crucial for designing efficient Flutter applications.