noun
stop button (on a bus)
Specifically the button passengers press to signal the driver to stop at the next bus stop.
次の停留所で降りるので、降車ボタンを押した。
I pressed the stop button because I'm getting off at the next bus stop.
降車 is the act of alighting from a vehicle; 降車ボタン is the specific button used for that purpose on a bus.
Compound of 降車 (こうしゃ, 'alighting from a vehicle') and ボタン ('button').