Automatic Movement
An automatic movement is a mechanical watch movement that winds itself from the wearer's wrist motion via an internal rotor.
An automatic movement is a mechanical watch movement that winds its own mainspring from the wearer's wrist motion. A weighted semi-circular rotor inside the case is mounted on a bearing and swings under gravity as the wrist moves; that rotation is geared into the winding train, tightening the mainspring without the wearer ever turning the crown.
Automatic movements were popularised in the 1920s-30s (Rolex's "Perpetual" rotor patent dates to 1931) and are now the dominant architecture for mid-range and luxury mechanical watches. Modern examples include the ETA 2824-2, Sellita SW200-1, Miyota 8215/9015, Seiko NH35/4R36/6R35, and every in-house caliber from Rolex, Tudor, Omega, and beyond.
Automatic movements still have a manual-winding capability via the crown for the first wear or after the watch has been sitting; once on the wrist, normal wear is usually enough to keep them at full power. Power reserves range from ~38 hours (older ETA 2824) to 70+ hours on modern calibers like the Tudor MT5612 or Omega Master Co-Axial.
Common questions
How often do you need to wind an automatic watch?
An automatic watch winds itself from wrist motion during normal wear. If it sits unworn for more than the power reserve (typically 38-70 hours) it will stop and need to be manually wound or shaken to restart.
Is automatic the same as mechanical?
Automatic is a type of mechanical movement. All automatic movements are mechanical, but not all mechanical movements are automatic — manual (hand-wound) mechanicals also exist.