1. To behave or to change in a particular way in response to or under the influence of some stimulus or prompting: When Lynn's friend told Mr. Todd what happened, he reacted with sadness.
2. To respond to something by showing the feelings or thoughts it arouses: Most people reacted in a positive way to the smoking ban in restaurants.
3. To respond to some notification or stimulus by taking action: The firefighters reacted quickly when they heard the alarm.
4. To respond to or to be affected by a medication, drug, food, air pollution, etc. and usually in a negative or bad way: Rose physically reacted and felt terrible after she took a dose of the medicine which was prescribed by Dr. Small.