Real Dictionary

button
countable noun: Buttons are small hard objects sewn on to shirts, coats, or other pieces of clothing. You fasten the clothing by pushing the buttons through holes called buttonholes.
verb: If you button a shirt, coat, or other piece of clothing, you fasten it by pushing its buttons through the buttonholes.

button in American English
any small disk, knob, etc. used as a fastening or ornament, as one put through a buttonhole on a garment
to fasten with or as with a button or buttons
noun: a small disk, knob, or the like for sewing or otherwise attaching to an article, as of clothing, serving as a fastening when passed through a buttonhole or loop

button in British English
noun: a disc or knob of plastic, wood, etc, attached to a garment, etc, usually for fastening two surfaces together by passing it through a buttonhole or loop