A concave surface curves inwards in the middle, like the inside of a bowl or a spoon. Light in a concave mirror or lens is gathered inward.

A concave surfaceConcave（凹的）表面 curves inwards in the middle中間向內彎，like the inside of a bowl or a spoon就像碗或湯匙的內側。Light in a concave mirror or lens凹面鏡或凹透鏡裡的光線 is gathered inward會向內聚集。

The everyday partner word is convex, which bulges out.

The everyday partner word is convex日常生活裡跟它相對的字是 convex（凸的），which bulges out是向外凸出的。

You use concave in maths, science, and careful talk about shape — bowls, dishes, some lenses, the inner side of a curve.

You use concave inConcave 用在 maths, science, and careful talk about shape數學、科學，以及仔細描述形狀的場合——bowls, dishes, some lenses, the inner side of a curve碗、盤子、某些鏡片、曲線的內側。

Though actually, people sometimes say concave when they only mean “hollow” or “dented,” which is close but looser: a dent is a local knock, while concave is a smooth inward curve.

Though actually不過其實，people sometimes say concave有時候人們說 concave when they only mean "hollow" or "dented,"只是想表達「hollow（中空）」或「dented（凹陷）」，which is close but looser意思接近但比較鬆散：a dent is a local knockdent 是局部撞出來的凹痕，while concave is a smooth inward curve而 concave 是平滑的向內彎曲。

As a noun, the concave can mean that inward-curving side, but the adjective is what you will meet most. A simple memory trick that also fits the history: concave is like a cave — it goes in.

As a noun當名詞用時，the concave can meanthe concave 可以指 that inward-curving side那個向內彎的那一側，but the adjective is what you will meet most但你最常遇到的還是形容詞的用法。A simple memory trick that also fits the history一個簡單又符合字源的記憶法：concave is like a caveconcave 就像 cave（洞穴）——it goes in是往裡面去的。