to do or say something because of what has happened or what someone else has done or said
When the fire alarm rang, all the students reacted quickly and left the building.
He didn't know how to react when he got the surprising news.
✦ From Latin re- "again" + agere "to do, drive", via French réagir.
(chemistry) to change and form something new when mixed with another substance
Vinegar and baking soda react to make bubbles of gas.
Iron reacts with oxygen and forms rust over time.
✦ Same origin as sense 1; the scientific use developed in the 19th century as chemistry advanced.
a process in which one or more substances change into new substances
Rust forming on iron is a slow chemical reaction.
The science teacher mixed vinegar and baking soda to show a bubbling reaction.
✦ extended scientific sense recorded in the 17th century as chemistry developed
a force equal in size and opposite in direction to another force
When you push on the wall, the wall pushes back with an equal reaction.
A rocket moves upward because the exhaust gases create a downward reaction.
✦ physics sense formalized in the 17th century by Isaac Newton
strong opposition to social or political change, favoring traditional ideas
The new law met fierce reaction from conservative groups.
History shows that progressive movements often face equal and opposite reaction.
✦ political sense arose in the early 19th century after the French Revolution to describe forces resisting change
