Watch terminology

Analog: The traditional dial; keeping time with hands.
Aperture: The date display window on a watch dial.
Calendar: Displays featuring the day, date or year in addition to the hour; analog watch dials show this feature in apertures or subdials.
Caliber: The configuration and size of the watch movement.
Countdown Timer: A chronograph function that measures how much of a preset period of time has passed.
Chronograph: A watch or clock with multiple functions measuring specific durations of time, often in fractions of a second. Subdials and hands measure the time periods; such as the stopwatch of a sports watch.
Chronometer: A high-precision timepiece whose movement has been quality-tested by the Controle Officiel Suisse des Chronometres [COSC], a Swiss laboratory. The COSC tests the movement at five different positions and 3 different temperatures for several consecutive days to determine accuracy. Timepieces qualifying as chronometers include a COSC certification number.
Digital: A digital watch shows the time and other features in a LCD (liquid crystal display) or LED (light emitting diode) display. This feature is useful displaying information on a multifunction watch.
Dual Time: A display that shows two time zones on the dial. The feature can have two dials, a subdial placed in the main dial, or analog and digital displays on the same watch.
Guilloche: A pattern of ridges that ripple outward from the center of a flat surface; a sunburst pattern. This texture is common on the dials of dress watches.
Jewels: The jewels form the bearings in a mechanical or automatic watch. The movement generally will have at least 17 jewels.
Kinetic: A watch mechanism or battery that is powered by natural movements of the wearer's arm. A quartz watch with kinetic movement never needs a new battery.
Lap Timer: A chronograph function that measures segments of a race; it can stop to show the time for each lap without losing track of the total race time.
Mechanical: Watch movement using a spring that must be wound by hand. The spring slowly unwinds to release the energy that powers the watch.
Moon Phase Dial: A subdial that tracks the phases of the lunar month. Some watches have a Sun and Moon subdial which tracks the 24-hour day.
Perpetual Calendar: Automatically resets the day at the end of the month or year, including leap years.
Power Reserve: The amount of energy, notated in hours, that a watch has stored in its movement. The average mechanical or automatic watch has a full power reserve of about 36 hours.
Skeleton:: This case design displays the watch movement with an open dial or with a clear crystal placed on the case back.
Sweep Hand: The marker that denotes the seconds as it moves around the dial of an automatic watch. Also called the sweep second hand, this marker moves in a smooth arc on the dial. The second hand of a quartz watch will click forward in second-long increments.
Tachymeter: A register set on the bezel that measures the distance covered over a specific period of time.
World Time: Found in digital watches, this function features a list of the current times in major cities around the world.