Other forms: reactions
In chemistry, a reaction is what happens when substances meet and change into something new. A chemical reaction, the reaction gives off heat, a violent reaction.
The everyday sense is a response to something: her reaction was immediate, public reaction, a knee-jerk reaction. In medicine, a reaction is an unwanted response to a drug or food — an allergic reaction — and that use is very common. In physics, Newton's third law pairs every action with an equal and opposite reaction. Note reactionary, which is political and means opposed to progress, and is not simply somebody who reacts.
1nounreactionThe change when substances meet and become something new.
“The reaction gives off enough heat to boil the water.”
2nounhappenedA response to something that has happened.
“Her reaction told him everything before she spoke.”
3nounharmfulA harmful response to a drug or food.
“He had a reaction to the antibiotic and came out in a rash.”
Too few real examples of this word to estimate which sense is commonest.
reaction is a response or change.
Latin re-, back, plus agere, to do or drive — acting back. Act, agent, agitate and reaction are all the same verb, and the physics sense stays closest to the Latin: a reaction is literally the doing that comes back at you.
Proto-Indo-European
*h₂eǵ-
Latin
reāctiō
Old French
reaction
Middle English
reaccion
English, today
reaction
Built from
Middle English reaccion, from Old French reaction, from Latin reāctiō, from the verb reagō, from re- (“again”) + agō (“to act”); more at re-, action; equivalent to react + ion.
Etymology from Wiktionary, CC BY-SA