A complication is any feature in a watch beyond the simple display of hours, minutes, and seconds, such as a calendar, chronograph, or moon phase indicator.
A chronograph is a stopwatch function built into a watch, allowing you to measure elapsed time. A chronometer is a watch that has been certified for exceptional accuracy by an independent testing institute like COSC. A watch can be both a chronograph and a chronometer.
Press the top pusher to start timing, press it again to stop, and press the bottom pusher to reset to zero. The central seconds hand measures elapsed seconds, while subdials typically show elapsed minutes and hours. Never operate the pushers while underwater on non-screw-down models.
In quartz watches, running the chronograph continuously drains the battery 2-3x faster. In mechanical watches, running the chronograph reduces power reserve by 20-40% due to increased energy consumption by the additional complications and friction in the movement.