- (countable & uncountable) A reaction is an action that happens because of another action.
- When he asked her to lunch, her first reaction was to say no.
- In part, McKenna sees the move to smaller cars as a natural reaction to rising gasoline prices.
- Fast reaction times are common in athletes.
- When oxygen is added, a chemical reaction begins and heat is released.
- A severe allergic reaction to the drug has killed five Americans.