What visual feature differentiates a radio button from other selection options?

The distinguishing visual feature of a radio button is its circular shape, which is reminiscent of old car radio buttons that would be pressed to select channels. This design not only makes it visually recognizable but also serves a functional purpose. When using radio buttons, users can select only one option from a set—all the other options will be automatically deselected when one is chosen. This differs from other selection options, such as checkboxes, which allow multiple selections, or dropdown menus, which provide a list of options to be selected from a single click. The circular design directly communicates this single-selection characteristic to users, making radio buttons an intuitive choice for such scenarios.