A classical or literary term for a crescent moon. In modern Japanese, 三日月 (みかづき) is the common word.
In old waka poems, the word 偃月 is sometimes used.
A 偃月刀 is a weapon with a crescent-moon-shaped blade.
From Chinese 偃月 (yǎnyuè), literally 'lying moon' or 'moon lying on its back', referring to the crescent shape. The exact historical derivation into Japanese is uncertain, but it is a classical Sino-Japanese term.