The moon when it appears as half-illuminated; also refers to anything shaped like a half-circle or semicircle.
From 'half' + 'moon' (Old English 'mona'). This compound has existed since Middle English to describe the lunar phase when the moon shows roughly half its face illuminated to Earth observers.
Halfmoon is one of those words that feels ancient because humans have watched the moon for millennia—Stonehenge watchers knew halfmoon phases! The simple compound perfectly captures what you see, showing how basic observation creates the most enduring vocabulary.
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