回應：對發生的事做出反應
核心畫面本身：事情一發生，你心裡或行為上「回」一下。
reaction (response) · noun · B2
The feeling or action an event or someone's words bring out of you.
反應、回應
核心畫面
Something happens TO you, and you act BACK. A reaction is the 'acting back' — the response that comes out of you (or out of a thing) because something first hit, was said, or was done. The prefix re- is 'back', so a reaction is always a back-action triggered by something before it.
可以想成一個前後動作：先有件事「打過來」（有人說了話、發生了狀況、兩種東西碰在一起），然後你或那個東西「回打過去」。…更多收起
可以想成一個前後動作：先有件事「打過來」（有人說了話、發生了狀況、兩種東西碰在一起），然後你或那個東西「回打過去」。reaction 就是那個「回應的動作」——因為前面先有了刺激，才有後面這一下回擊。字首 re- 是「回」，所以 reaction 永遠是「被前面某件事觸發、再反過來」的動作。心理上的反應、過敏、化學反應、反作用力、政治上的反彈，全都是這個「先有來、才有回」的畫面。From the word’s history
re- 'back' + action: literally a 'back-action', a response that goes back against what acted first.
reaction 由 re-（回、反）＋ action（動作）組成，字面就是「回過去的動作」，也就是被前面某件事觸發後反打回去的反應。
Online Etymology Dictionary (etymonline.com), 'reaction' (re- 'back' + action)
Four branches
身體的反應：物質進到體內引起的回應
把「回應」用在身體上：某種食物或藥物進來，身體「回」以不舒服。
reaction (bad physical response, allergy) · noun · B2
A bad effect on your body caused by a food, drug, or other substance that makes you ill.
（過敏）反應、不良反應
科學上的反應：物質或力的回打
把「有來、才有回」搬進科學：物質相遇而生變、或一個力必引出反方向的回力。
reaction (chemical) · noun · B2
Two or more substances mixing together and turning into something chemically different.
化學反應
反動：對舊作風或變化的反彈
把「回應」放大到社會：一群人因為不滿先前的做法或變化，而集體「反過來」。
a reaction (backlash against how things were) · noun · B2
A change in people's attitudes or behaviour because they dislike how things were done before.
（對舊事物的）反彈、反動潮流
原始詞典資料5 筆記錄
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
reactionnoun/ɹiˈækʃən/
- 1
An action or statement in response to a stimulus or other event.
- The announcement of the verdict brought a violent reaction.
- You were in the courtroom. What is your reaction?
- 2
chemistrynatural-sciencesphysical-sciencesA transformation in which one or more substances is converted into another by combination or decomposition.
- In this reaction, the acid and base will neutralize each other, producing a salt.
- 3
governmentpoliticsReactionary politics; a period in which reactionary thought or politics is resurgent or dominant.
- And as Europe advanced year by year more deeply into the low featureless plains of reaction, censorship and mediocrity and the pestilential swamps of poverty, unhappiness and oppression, the image of the liberating revolution became ever more luminous.
- 4
economicssciencesUnpurposeful behavior.
- 5
An icon or emoji appended to a posted message by a user to express their feeling about it.
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。