It means "the entire sky. " Not just the patch above your head. All of it, horizon to horizon, the full celestial dome. You won't hear this one in everyday conversation. It belongs to the language of astronomers, planetariums, and weather stations. When scientists map every visible star or measure cloud cover across the whole sky, 全天 (zenten) is the word they reach for. In daily Japanese, you'd say 空一面に (sora ichimen ni), "all across the sky. "

But 全天 (zenten) carries a weight those casual phrases don't. 全 (zen) means "whole" or "complete. " Its exact origin is debated, but scholars agree the character is tied to jade (玉): the idea of a flawless, unbroken gemstone. Pure jade, with nothing missing. That image of perfection became the meaning: complete, entire, whole. 天 (ten) is "heaven" or "sky. " A single line (一) stretching above a standing person (大): the vast expanse over our heads. This one shows up everywhere in Japanese.

Put them together and you get something almost overwhelming. The whole sky at once. Here's the word in a sentence: 昨夜は空気が澄んでいて、星が全天に広がって見えた。 Sakuya wa kūki ga sunde ite, hoshi ga zenten ni hirogatte mieta. Last night the air was clear, and the stars looked spread across the entire sky. Related words worth knowing: - 天空 (tenkū) - the sky, the heavens - 星空 (hoshizora) - starry sky - 夜空 (yozora) - night sky - 天気 (tenki) - weather - 全部 (zenbu) - everything, all - 全然 (zenzen) - not at all