noun
Commonly used for a visible or social response to something someone says or does, often with emphasis on how the response looks or feels.
彼のリアクションが大げさで、みんな笑った。
His reaction was so exaggerated that everyone laughed.
この投稿にはたくさんのリアクションが
There were many responses to this post.
noun
Physics use for a reaction, especially as a counterpart to an action; in ordinary Japanese, technical terms such as 反作用 or 反力 may be more specific depending on context.
物理の文脈では、作用に対するリアクションを反作用と呼ぶ。
In a physics context, the reaction to an action is called a reaction force.