Definition of react verb from the Oxford Advanced Learner's Dictionary

react

verb

riˈækt

riˈækt

1 [intransitive] react (to something) (by doing something) to change or behave in a particular way as a result of or in response to somethingLocal residents have reacted angrily to the news.I nudged her but she didn't react.You never know how he is going to react.The market reacted by falling a further two points.

2 [intransitive] (+ adverb/preposition) to become ill/sick after eating, breathing, etc. a particular substancePeople can react badly to certain food additives.3 [intransitive] react (with something)| react (together) (chemistry) (of substances) to experience a chemical change when coming into contact with another substanceIron reacts with water and air to produce rust.When an acid and an alkali react together the result is a salt and water.

Phrasal verbs

react against somebody/something

to show dislike or opposition in response to something, especially by deliberately doing the opposite of what somebody wants you to doHe reacted strongly against the artistic conventions of his time.