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)