Lume
Lume is the glowing material on the hands and markers that charges in light and glows in the dark, so you can read the watch at night. The most common types are Super-LumiNova and Seiko's LumiBrite.
Lume is not radioactive; it absorbs daylight or artificial light and releases it for a while in the dark. Its quality is measured by brightness and duration: good lume is still legible hours after leaving the light. Seiko's LumiBrite is strong in this class and gives dive watches a clear advantage. On some budget watches the lume is weaker or unevenly applied; if night or dive use matters to you, lume strength is a criterion worth checking.