1the inside or interior of something — the inner space enclosed by the object, not visible from outside.
箱の中に本が三冊入っている。
There are three books inside the box.
部屋の中は思ったより静かだった。
Inside the room was quieter than I expected.
カバンの中を探したが、鍵は見つからなかった。
I searched inside my bag, but couldn't find the keys.
冷蔵庫の中には何も残っていなかった。
There was nothing left inside the fridge.
2among or within a group, range, or set of options. Typically used in the patterns 〜の中で / 〜の中から to single out one item from many or to restrict the scope.
クラスの中で彼が一番背が高い。
He's the tallest in the class.
三つの案の中から一つを選んでください。
Please choose one from among the three proposals.
知っている人の中で一番頼りになるのは彼女だ。
Of the people I know, she's the most reliable.
3the middle or central part — the position at or near the center of something.
町の中を大きな川が流れている。
A big river runs through the middle of the town.
テーブルの中ほどに花瓶を置いた。
I placed the vase around the middle of the table.
夜中に目が覚めて、しばらく本を読んでいた。
I woke up in the middle of the night and read a book for a while.
会場の中ほどの席が空いていた。
The seats around the middle of the venue were empty.
4in the midst of (a situation or condition). Used in patterns like 〜の中で / 〜の中(を) to describe something happening *within* a circumstance. Distinct from the suffix 〜中 read ちゅう (e.g., 会議中); the なか version has a more descriptive, literary feel.
雨の中を傘もささずに歩いてきた。
He came walking through the rain without an umbrella.
忙しい中、来てくれてありがとう。
Thank you for coming despite being so busy.
拍手の中、彼は壇上に上がった。
Amid the applause, he stepped up onto the stage.
暮らしの中で日本語を少しずつ覚えていった。
I picked up Japanese little by little in the course of daily life.
5an interval or gap between two things — a somewhat limited and slightly old-fashioned usage; 間 (あいだ) is more common in modern Japanese.
二人の中をとりもつ人が必要だった。
Someone was needed to mediate between the two of them.
6the middle ground or average between two extremes. Mainly survives in the idiom 中をとる (to compromise / pick the middle path). Different from the size/grade 中 read ちゅう; this one is read なか.
両者の意見の中をとって、結論を出した。
We split the difference between the two sides' opinions and reached a conclusion.
7the middle one (second of three), as in 中の子 (the middle child of three siblings) — a somewhat old-fashioned way to refer to a person in the middle position. Note that for sequences like 上巻・中巻・下巻 (upper, middle, lower volumes), 中 is read ちゅう, not なか.
三人兄弟の中の子は、おとなしい性格だ。
The middle child of the three siblings has a quiet personality.
8the middle grade in an upper/middle/lower (上・中・下) ranking — used historically when describing quality tiers; pronounced なか. In modern standalone uses such as 中サイズ or 中くらい, the reading is normally ちゅう, not なか.
品物を上・中・下に分け、中のものを選んだ。
We sorted the goods into upper, middle, and lower grades and chose one of the middle grade.
9(archaic) a colloquial term for a licensed pleasure quarter in the Edo period — used in expressions like 廓の中 or 中に通う when referring to places such as Yoshiwara. Not used in modern everyday Japanese.