plural stimuli, 1680s, originally as a medical term, "something that goads a lazy organ" (often the male member), from Modern Latin stimulus "goad" (see stimulation). General sense is from 1791. Psychological sense is first recorded 1894.
stimulus stim·u·lus (stĭm'yə-ləs)
n. pl. stim·u·li (-lī')
That which can elicit or evoke an action or response in a cell, an excitable tissue, or an organism.
Plural stimuli (stĭm'yə-lī')
plur. stimuli (stim-yuh-leye)
An action, condition, or person that provokes a response, especially a conditioned response.