re·ac·tion (r?-?k?sh?n)n.1. a. A response to a stimulus.b. The state resulting from such a response.2. A reverse or opposing action.3. a. A tendency to revert to a former state.b. Opposition to progress or liberalism; extreme conservatism.4. Chemistry A change or transformation in which a substance decomposes, combines with other substances, or interchanges constituents with other substances.5. Physics A nuclear reaction.6. Physics An equal and opposite force exerted by a body against another body that is exerting a force on it.7. The response of cells or tissues to an antigen, as in a test for immunization.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.reaction (r??æk??n) n1. a response to some foregoing action or stimulus2. the reciprocal action of two things acting together3. (Government, Politics & Diplomacy) opposition to change, esp political change, or a desire to return to a former condition or system4. (Psychology) a response indicating a person’s feelings or emotional attitude5. (Medicine) med a. any effect produced by the action of a drug, esp an adverse effect. Compare side effectb. any effect produced by a substance (allergen) to which a person is allergic6. (Chemistry) short for chemical reaction, nuclear reaction7. (Nuclear Physics) short for chemical reaction, nuclear reaction8. (Stock Exchange) stock exchange a sharp fall in price interrupting a general rise re?actional adjUsage: Reaction is used to refer both to an instant response (her reaction was one of amazement) and to a considered response in the form of a statement (the Minister gave his reaction to the court’s decision). Some people think this second use is incorrectCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014re?ac?tion (ri?æk ??n) n. 1. action in response to some influence, event, etc.: the nation’s reaction to the president’s speech. 2. a. a physiological response to an action or condition. b. a physiological change indicating sensitivity to foreign matter: an allergic reaction. 3. an action in a reverse direction or manner. 4. a movement toward extreme political conservatism; a desire to return to an earlier system or order. 5. a. the reciprocal action of chemical agents upon each other; chemical change. b. a process that, unlike a chemical reaction, has the power to change the nucleus of an atom, as radioactive decay, fission, or the like. 6. Mech. the instantaneous response of a system to an applied force, manifested as the exertion of a force equal in magnitude, but opposite in direction, to the applied force. [1635?45; on the model of react] re?ac?tion?al, adj. Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.re·ac·tion (r?-?k?sh?n)1. A rearrangement of the atoms or molecules of two or more substances that come into contact with each other, resulting in the formation of one or more new substances. Chemical reactions are caused by electrons of one substance interacting with those of another. The reaction of an acid with a base, for example, results in the creation of a salt and water. Some, but not all, reactions can be reversed.2. A change to the structure of an atomic nucleus; a nuclear reaction.3. An action that results directly from or counteracts another action, especially the change in a body’s motion as a result of a force applied to it. Some reactions counteract forces and are not readily apparent. When an object rests on a surface, such as a table, for example, the downward force it applies to the surface is counteracted by an equal but upwards force, or reaction, applied by the surface. See more at Newton’s laws of motion.The American Heritage® Student Science Dictionary, Second Edition. Copyright © 2014 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.reactionA process in which substances react to form new substances. Bonds are broken and re-formed in chemical reactions.Dictionary of Unfamiliar Words by Diagram Group Copyright © 2008 by Diagram Visual Information Limited