Of a collar, able to be buttoned down to the shirt, as over a necktie. / (idiomatic) Serious; staid; businesslike. / (idiomatic) Conservative; conventional; unimaginative.

A shirt with a button-down collar

A button on a corset.

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

A button on an alarm clock which, when pressed, silences the alarm temporarily, granting a few more minutes of sleep before the alarm sounds again.

(transitive) To fasten with a button or buttons. / (intransitive) To fasten all the buttons on a coat, or similar item of clothing, to keep warm.

(simile) Very cute.

(simile) Very bright (clever, smart, or crafty) / (simile) Very cheery, energetic and upbeat.

An emergency button which, when pressed, alerts authorities of danger or triggers an alarm system.

present participle of button up