A radio button is a graphical user interface element that allows users to select one option from a set of predefined choices. It is typically represented as a small circle that can be filled in when selected. When a user clicks on a radio button, any previously selected button in the same group is automatically deselected, ensuring that only one option can be chosen at a time. Radio buttons are commonly used in forms and surveys, where users need to make a single choice among multiple options, such as selecting a preferred color or answering a question. They are part of the broader category of user interface controls, which also includes checkboxes and dropdown menus.