Reaction (noun)

1. A response to a particular event, situation, or stimulus.
2. A change or effect that results when two or more substances interact.
3. Opposition to a social or political development or trend.
4. A physiological response to a stimulus.

Origin:

Late 16th century (in the sense "a reverse action or tendency"): from french réaction or late latin reactio(n-), from re- "back" + actio(n) "a doing". the term originally denoted an action in response to a preceding one, and has not changed its meaning.

Examples:

1. The crowd's reaction to the speech was mixed.
2. The chemical reaction produced heat and light.
3. The reaction to the news was shock and dismay.
4. He had an allergic reaction to the medication.
5. The government's reaction to the crisis was slow.