Definition
A nonmetallic chemical element (symbol Se) whose electrical resistance changes with the amount of light striking it. This light-sensitive property is used in photoelectric cells and rectifiers found in some aircraft instruments and electrical components.
Plain English
Selenium is a material that conducts electricity better when light shines on it. That makes it useful in small electrical parts that need to react to light or convert current from one form to another.
Context Anchor
Seen in aircraft maintenance discussions of older electrical equipment, light-sensitive devices, and selenium rectifiers.
Derivation
From the Greek selene, meaning 'moon.' The element was named by analogy to tellurium (named for the earth), and the moon connection is a clue that selenium is a light-related material — fitting given its light-sensitive electrical behavior.
Why Pilots Care
Selenium rectifiers provided reliable AC-to-DC conversion in pre-silicon aircraft power supplies before modern diodes became standard.
Intuition Check
Selenium here is not a vitamin supplement or a brand name. In this context, it means the chemical element used as a material inside certain electrical devices.
Example Sentence 1
The older battery charger used a selenium rectifier to convert AC power into the DC current required by the aircraft system.
Example Sentence 2
Older aircraft often used selenium rectifiers to supply direct current to the battery charging system.