Webster's English Dictionary

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stimulus

stim.u.lus \'stim-y*-l*s\ \-.li-, -.le-\ n or stim.u.li [L] pl  : something 
   that rouses or incites to activity : as  : STIMULANT  : an agent (as an 
   environmental change) that directly influences the acti vity of living 
   protoplasm (as by exciting a sensory organ or evoking muscular contraction 
   or glandular secretion)