A button was an object used to fasten clothing.
The First Doctor complained to Aldridge that humans put buttons in the most awkward places. (PROSE: A Big Hand for the Doctor)
When the Doctor, Ian Chesterton and Barbara Wright were miniaturised, they hid from a bird under a yellow button. (COMIC: Dr Who in "Lilliput")
Ala ud-Din had never seen buttons before meeting the Fourth Doctor and was fascinated by them. (PROSE: The Nobility of Faith)
The Third Doctor and Liz Shaw were assigned to work on a UNIT uniform button with a homing device. (COMIC: The Fishmen of Carpantha)