last quarter moon; third quarter moon; waning half-moon
Astronomical term for the moon phase when the left half is illuminated (in the Northern Hemisphere). Often contrasted with 上弦 (first quarter moon).
See also: 上弦
Tonight we can see the last quarter moon.
下弦の月は明け方に南の空に見える。
The last quarter moon is visible in the southern sky around dawn.
From 下 (lower) + 弦 (bowstring, crescent), referring to the moon's shape resembling a bowstring in its lower (waning) phase.