noun
continuously burning light; eternal flame
Typically refers to a lamp or candle kept perpetually lit at a Buddhist altar or memorial. The term is somewhat specialized and may not be widely known outside religious contexts.
お寺の本堂には常灯明がともされている。
A continuously burning light is kept lit in the temple's main hall.
仏壇の常灯明を絶やさないように気をつけている。
I make sure the eternal flame on the Buddhist altar never goes out.
灯明 is a general term for a light offered to a deity or Buddha, while 常灯明 specifically emphasizes that it is kept burning continuously.
Compound of 常 (jō, 'constant') + 灯明 (tōmyō, 'sacred light'). The exact historical derivation is uncertain, but the word is conventionally associated with Buddhist practice.