Chronograph

A chronograph is stop-watch. The pushers on the side are a good indicator that a watch is a chronograph watch.

Helium Release Valve

This valve is for professional divers and helps release the trapped inert  gas that builds up in a watch during prolonged dives of 80 m or more, specifically saturation diving.

Luminosity

Many watches have hour markers and hands that glow-in-the-dark based on different technologies.

Pusher/Button

This is an attachment to the case that would control some watches functions, such as chronograph.

Subdial

Sometimes a smaller dial is inside the main dial. It shows information that isn’t provided by the main dial, such as dates or seconds.

Tachymetre

This is a feature on a watch that allows the user to measure speed. Some also allow the user to measure distance or fuel economy.

Unidirectional Rotating Bezel

This is a part of a dive watch that helps track air supply. It moves only in a counter-clockwise direction and as a result, helps a diver time how much time they have spent under water.

Exhibition case back

An exhibition case back is transparent, so you can see the mechanical movement.

Jewel

These are small sapphires or rubies used as bearings for the gears to reduce friction.

 Power Reserve Indicator

This indicates how much potential energy is left in a watch. Some watches can have up to a 10-day power reserve.