Commonly used for a visible or social response to something someone says or does, often with emphasis on how the response looks or feels.
His reaction was so exaggerated that everyone laughed.
There were many responses to this post.
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.
In a physics context, the reaction to an action is called a reaction force.
Seeing her reaction, I realized it was a joke.
He reacted to the news right away.
It was a surprise, but everyone's reaction was underwhelming.
He always reacts in an exaggerated way.