A stimulus is anything that can trigger a physical or behavioral change. The plural of stimulus is stimuli.
Stimuli can be external or internal.
An example of external stimuli is:
Your body responding to a medicine
An example of internal stimuli:
Your vital signs change due to a change in the body
Linda J. Vorvick, MD, Medical Director and Director of Didactic Curriculum, MEDEX Northwest Division of Physician Assistant Studies, Department of Family Medicine, UW Medicine, School of Medicine, University of Washington. Also reviewed by A.D.A.M. Health Solutions, Ebix, Inc., Editorial Team: David Zieve, MD, MHA, David R. Eltz, Stephanie Slon, and Nissi Wang.