A crown is a rotating wheel typically located at the side of a watch case.
Usually located at the 3:00 position on the dial, the crown is a typically a fluted wheel used to set the time. With the advent of keyless works, the crown may also be pulled out to set other indicators such as day, date, dual time-zone, seconds and so on, and with manual winding watches for tensioning the mainspring. Crown winding was first used in 1844 for pocket watches, with the time set by manually moving the hands on the (open) dial.
Some watches feature a screw-in crown which can seal the watch against water and dust. Some also feature multiple crowns for various features, along with pushers, sliding switches, and other controls.