bottom, top
The top button of his shirt was undone.

coat, shirt, etc.
brass, pearl, etc.

button (AmE), do up (BrE), fasten
unbutton (AmE), undo (BrE), unfasten
fumble with
lose
My coat has lost a button.

sew on

be missing, come off
There was a button missing from his shirt.

on, start
off, stop
control
fast-forward, pause, play, rewind
delete, mute, reset, snooze, etc.
alarm, panic
She hit the alarm button as fast as she could.

elevator (AmE), lift (BrE)
intercom
mouse
self-destruct (often figurative)

click (computing), depress, hit, hold down, press, punch, push, tap
Click the left mouse button twice.

release

at the touch of a button
The remote control allows you to change channel at the touch of a button.

have your finger on, keep your finger on

lapel
She wore a ‘Vote Yes’ lapel button.

campaign
The candidates all distributed campaign buttons and bumper stickers.

peace
He carries around an old backpack with peace buttons on it.

wear
They all wore buttons saying ‘Stop the war’.