What is Button ?
Button is (noun) 1. a little round disc for fastening clothes that you push through a buttonhole The wind is cold do up the buttons on your coat. A button’s come off my shirt. 2. a little round disc which you push to operate something such as a bell Press the ‘up’ button to call the lift. Push the red button to set off the alarm. ((verb)) to fasten with buttons He buttoned (up) his coat because it was cold.
source: Easier English, Student Dictionary Upper Intermediate Level