Lume
Lume is the glowing material applied to the hands and markers; it charges in light and glows in the dark, letting you 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 gives it back for a while in the dark. Its quality is judged 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 edge. On some budget watches the lume is weaker or applied unevenly; if night or dive use matters to you, lume strength is a criterion worth checking.