noun
flag signaling; flag signalling
Refers to communication using flags, typically in maritime or military contexts.
船は旗旒信号でメッセージを送った。
The ship sent a message by flag signaling.
Compound of 旗旒 (kiryū, 'flag') and 信号 (shingō, 'signal'). The exact derivation is straightforward; the term is technical.