re- + act

source: [振り向いたシンジの顔がどんどん赤くなるＧＩＦ画像｜無料GIF画像検索 GIFMAGAZINE 132446]

respond or behave in a particular way in response to something:

«…に/…することによって» 反応する, 対応する(respond) «to/by doing»

e.g. he reacted angrily to the news of his dismissal

e.g. the market reacted by falling a further 3.1%.

(react against) respond with hostility, opposition, or a contrary course of action to:

〈人が〉 【規則･規範などに】反抗する, 反発する(rebel) «against»

e.g. they reacted against the elite art music of their time.

(of a person) suffer from adverse physiological effects after ingesting, breathing, or touching a substance:

〈人が〉容態が悪くなる; 【薬･食物などに】反応する «to»

e.g. many babies react to soy-based formulas.

Chemistry & Physics interact and undergo a chemical or physical change:

e.g. the sulfur in the coal reacts with the limestone during combustion.

with object Chemistry cause (a substance) to undergo a change by interacting with another substance.

〘化〙 〈(2つの)物質が〉 «別の物質と/…に» 化学反応を起こす «with/on»

Stock Market (of stock prices) fall or rise in reaction to events, developments, etc.

〈価格･市場が〉 【事件･ニュースなどに】反応する «to»

mid 17th century: from re- (expressing intensive force or reversal) + act, originally suggested by medieval Latin react- ‘done again’, from the verb reagere.