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reactive

[ree-ak-tiv]
adjective
  1. tending to react.
  2. pertaining to or characterized by reaction.
  3. Electricity. pertaining to or characterized by reactance.

Origin of reactive

First recorded in 1705–15; react + -ive
Related formsre·ac·tive·ly, adverbre·ac·tive·ness, nounan·ti·re·ac·tive, adjectivehy·per·re·ac·tive, adjectivenon·re·ac·tive, adjectiveo·ver·re·ac·tive, adjectiveun·re·ac·tive, adjective
Can be confusedproactive reactionary reactive
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for overreactive

reactive

adjective
  1. readily partaking in chemical reactionssodium is a reactive metal; free radicals are very reactive
  2. of, concerned with, or having a reactance
  3. responsive to stimulus
  4. (of mental illnesses) precipitated by an external causereactive depression
Derived Formsreactively, adverbreactivity (ˌriːækˈtɪvɪtɪ) or reactiveness, noun
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
Word Origin and History for overreactive

reactive

adj.

1712, from react + -ive. Related: Reactively; reactiveness; reactivity.

Online Etymology Dictionary, © 2010 Douglas Harper
overreactive in Medicine

reactive

[rē-ăk′tĭv]
adj.
  1. Tending to be responsive or to react to a stimulus.
  2. Characterized by reaction.
  3. Tending to participate readily in chemical reactions.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.