Archaic literary term for the break of day. In modern Japanese, 夜明け (よあけ) or 明け方 (あけがた) are the standard words.
In classical texts, 昧旦 was used as a word for dawn.
A Sino-Japanese compound of 昧 (dark, obscure) and 旦 (daybreak). The exact historical derivation is uncertain; it appears in classical Chinese and was adopted into literary Japanese.