A corrector is a flush or inset button used to correct an internal complication in a watch, typically a calendar.
Most watches use a crown to control timekeeping functions, and many other complications are activated by means of a pusher. But complications like a calendar are typically adjusted by means of a button that sits flush to the case, called a corrector. These buttons often have an indent and sometimes a corrector tool or pusher tool is included to safely depress them.
Although all correctors are buttons, not all buttons are correctors. If a button stand out from the case, it is typically called a pusher instead. But some watches, such as the A. Lange & Söhne Lange 1, do feature correctors that resemble pushers, standing out from the case and easily pressed.