Wiktionary
n. 1 Any of a line of preset buttons on a car radio used to select the station to be listened to; pressing one releases the others 2 (context graphical user interface English) Any of a group of widgets in a graphical user interface that is used to select one of a group of options; the selection of any one deselects the others
Wikipedia
A radio button or option button is a graphical control element that allows the user to choose only one of a predefined set of options.
Radio buttons are arranged in groups of two or more and displayed on screen as, for example, a list of circular holes that can contain white space (for unselected) or a dot (for selected). Each radio button is normally accompanied by a label describing the choice that the radio button represents. The choices are mutually exclusive; when the user selects a radio button, any previously selected radio button in the same group becomes deselected (making it so only one can be selected). Selecting a radio button is done by clicking the mouse on the button, or the caption, or by using a keyboard shortcut.
It is possible that initially none of the radio buttons in a group are selected. This unselected state cannot be restored by interacting with the radio button widget, though it may be possible through other user interface elements. When used in an HTML form, if no button in a group is checked, then no name-value pair is passed when the form is submitted. For example, for a radio button group named Sex with the options Male and Female, the variable Sex would not be passed, even with a blank value.
Compare to a checkbox, which allows more than one (or no) item to be selected and for the unselected state to be restored.
Usage examples of "radio button".
Instead, he keyed the radio button in a series of long and short squelch breaks to signal that he was okay.
He took his hand off the radio button before he could say something else.
Filippov was excitedly pressing his radio button that rang his aide's radio--there was no answer.
Lucas followed, pressed the radio button, trying to talk through the bumps.
Edgar Rice Burroughs, Katsuhiko Shimoda, reached above Ben Cassidywho was scrunched on the floor behind the seatsand flipped a switch to automatically seal the hatch while Boggs stabbed the radio button with his gauntleted thumb.
I held her into the climbing turn, pressing the radio button with the left thumb.
Slowly, without taking his attention from the landscape or his finger from the trigger, the man lowered his free hand and touched the radio button on his wrist.