名詞 wane:

1. a gradual decline (in size or strength or power or number)
   同義詞：ebb, ebbing

動詞 wane:

1. grow smaller
   同義詞：decline, go down
2. become smaller
3. decrease in phase

1. The empire was on the wane after its period of prosperity.
   帝國經過全盛以后逐漸衰落。
2. The moon waxes and wanes every month.
   月亮每個月都有盈虧。
3. By now the power of the Roman Empire was on the wane.
   到這時，羅馬帝國已日益衰敗。

wane

v.intr.（不及物動詞）
waned,wan.ing,wanes
To decrease gradually in size, amount, intensity, or degree; decline.
衰落：體積、數量、密度或程度的逐漸減??；衰退
To exhibit a decreasing illuminated area from full moon to new moon.
（月）虧，缺：從滿月到新月期間表現出逐漸減小的明亮區(qū)
To approach an end.
結束：到達終點
n.（名詞）
The act or process of gradually declining or diminishing.
衰退：逐漸沒落或減小的行為或過程

A time or phase of gradual decrease.
衰退期：逐漸衰退的時間或階段
The period of the decrease of the moon`s illuminated visible surface.
月虧期：月亮的可見的明亮部分逐漸變小的時期
A defective edge of a board caused by remaining bark or a beveled end.
缺角方木：方木的由剩余樹皮或斜角末端引起的缺損的邊緣

<習慣用語>on the wane
In a period of decline or decrease:
衰減：處于正在衰退或減少的時期：
例句：
.The tide was near the turn and already the day was on the wane.(James Joyce)
.潮水即將退去，白晝漸漸消逝.(詹姆斯·喬伊絲)

來源：
Middle English wanen
中古英語 wanen
from Old English wanian * see eu- 2
源自 古英語 wanian *參見 eu- 2