1) A pathological process resulting from the reaction of the body to external forces and abnormal conditions that tend to disturb the organism's homeostasis.

2) Refers to the emotional, psychological, or physical effects as well as the sources of agitation, strain, tension, or pressure. Used for both human and animal populations.

3) The biological response to noxious, demanding, or unpleasant stimuli or conditions; do not confuse stress with stressor, which is the stimulus.