1The sky — the open space of air above the earth, where you see blue sky, clouds, and stars.
空に大きな虹がかかっていた。
A large rainbow stretched across the sky.
星がきれいに見える夜は空を見上げたくなる。
On nights when the stars are beautiful, I feel like looking up at the sky.
飛行機が真っ青な空を飛んでいく。
The airplane flies through the deep blue sky.
2Refers to the look of the sky as an indication of weather, as in 怪しい空 ("threatening-looking sky").
空が怪しくなってきたので、傘を持って出かけた。
The sky started looking ominous, so I took an umbrella with me.
3An archaic way of saying "a distant place" or "the beyond," found in classical poetry such as waka. Essentially unused in modern conversation.
4Used in the form 〜空もない to mean there isn't even room or time (mentally) to do something.
忙しくて休む空もない一週間だった。
It was a week so busy there wasn't even a moment to rest.
5Used as 空で to mean reciting or saying something purely from memory, without looking at anything written.
その詩なら空で言えるくらい何度も読んだ。
I've read that poem so many times I can recite it from memory.
彼女は取引先の電話番号を全部空で覚えている。
She has all the clients' phone numbers memorized.
6An archaic sense meaning "falsehood" or "a lie." In modern Japanese, 嘘 or 虚偽 are used instead, and 空 is no longer used with this meaning.
7A prefix attached before adjectives to add a vague, unaccountable nuance of "somehow ~," as preserved in words like 空恐ろしい ("eerily frightening, for no clear reason").
静まり返った夜道を歩くと空恐ろしい気持ちになる。
Walking down the dead-silent night street, I feel an eerie, nameless dread.
8A prefix attached before a noun or verb to add the meaning of "fake" or "insincere," as preserved in words like 空世辞 ("empty flattery") and 空涙 ("crocodile tears").
彼女は上司の前で空涙を流して見せた。
She put on a show of fake tears in front of her boss.