Meaning of REACTION in English
1》 an instance of reacting to or against something.
↘( ~s ) a person's ability to respond physically and mentally to external stimuli.
2》 opposition to political or social progress or reform.
3》 a process in which substances interact causing chemical or physical change.
4》 Physics a force exerted in opposition to an applied force.
~ist noun & adjective
Concise Oxford English vocab. Сжатый оксфордский словарь английского языка. 2004