Button (noun)

1. A small, usually round object that is used to fasten clothes and is pushed through a buttonhole.
2. A small, usually round, object that is used to operate a device, such as a computer mouse or a telephone.
3. A small, usually round, object that is used to make a selection on a computer screen or other electronic device.

Origin:

Middle english button, from old french bouton, from old frankish *buttōn, from latin buttō, from butta knob, from greek boûton.

Examples:

1. He buttoned his shirt.
2. The computer mouse has two buttons.
3. He pressed the button to turn on the radio.
4. She buttoned her coat before going outside.
5. The elevator had buttons for each floor.