1act in response to something; respond in a particular way:he reacted angrily to the news of his dismissalthe market reacted by falling a further 3.1%
(react against) respond with hostility or a contrary course of action to:they reacted against the elite art music of their time
suffer from adverse physiological effects after ingesting, breathing, or touching a substance:many babies react to soy-based formulas
Stock Exchange (of share prices) fall after rising:the shares reacted to 222p before rallying to 228p
2 Chemistry & Physics interact and undergo a chemical or physical change:the sulphur in the coal reacts with the limestone during combustion
[with object] cause (a substance) to undergo a chemical or physical change by interacting with another substance:in the new process the ammonia will be reacted with oxygen
