Because the term “stress” has become so broadly and variably used in biology, some initial distinctions are necessary. First, the “stress” construct originates from material science, where it denotes an amount of external force, or load, that produces a corresponding measure of internal deformation, or “strain” In its expansion to biology, this distinction has been lost, and the term stress is applied both to the external forces that challenge the organism, and the internal processes that result. Here, we will reserve the term “stress” and “stressors” for external demands placed on the organism. Second, “strain” in material science is a passive deformation. In contrast, biological organisms respond to external demands with a highly dynamic combination of physiological, emotional, cognitive and behavioral responses that have evolved to be adaptive, although they may be more or less successful in a given instance. In the short term, re-establishing a preexisting equilibrium, or homeostasis, is the classical example of a successful adaptive response, but this class is clearly broader. For instance, eliminating the challenge altogether by moving away from it is an equally successful
