Compression is the term for the reduction in volume that stress causes in any object or material. Living systems as well as solids, liquids, and gases may all endure compression. For example, 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. In the latter, compression is measured against the system's volume at the standard pressure to which an organism is subjected.