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unconditioned stimulus

British  

noun

  1. psychol any stimulus evoking an unlearnt response, esp in the context of classical conditioning, in which the conditioned stimulus is followed by the unconditioned one

"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

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Note that most English-language textbooks use the terms “unconditioned stimulus,” “unconditioned response,” and so on.

From Scientific American