compression, decrease in volume of any object or substance resulting from applied stress. Compression may be undergone by solids, liquids, and gases and by living systems. In the latter, compression is measured against the system’s volume at the standard pressure to which an organism is subjected—e.g., the pressure of the atmosphere at sea level is the standard, or reference, for most land animals, but the standard for deep-sea fishes and similar specialized forms is the normal pressure of their environment.