Reaction or response in a general sense — a reply, feedback, or change in behavior shown by a person or thing.
彼の冗談に対する彼女の反応は笑顔だった。
Her reaction to his joke was a smile.
新製品の発表に、SNS上でさまざまな反応が寄せられた。
Various reactions appeared on social media after the new product announcement.
先生は生徒の反応を見ながら説明を進めた。
The teacher proceeded with the explanation while watching the students' reactions.
A physical or biological reaction to a stimulus, such as an allergy or side effect observed in an organism.
薬を飲んだ後、皮膚に異常な反応が出た。
After taking the medicine, an abnormal skin reaction appeared.
花粉に対する反応で目がかゆくなった。
I got itchy eyes from an allergic reaction to pollen.
新しい化粧品を試して、顔に反応が出なかったので安心した。
I tried the new cosmetics and was relieved that I had no reaction on my face.
In chemistry, a reaction — an interaction between substances that produces change or products (chemical reaction).
酸と塩基の反応で塩と水が生成した。
An acid-base reaction produced salt and water.
この化学反応は温度によって速さが変わる。
The rate of this chemical reaction changes depending on temperature.