From Greek poikílos - different, changeable and osmós - push, pressure.

Poikiloosmic organisms - organisms whose internal environment osmotic pressure (blood, lymph, coelomic vein) depends on the osmotic pressure of the external environment, i.e. is in osmotic equilibrium with the external environment.

Poikilosmotic animals, aquatic animals unable to maintain more or less constant Osmotic pressure of cavity and tissue fluids when water salinity changes. The osmotic pressure of their internal environment is equal to or slightly higher than the external pressure. These include lower invertebrates, bivalves, many ringworms, echinoderms, and others. Poikiloosmic organisms, unlike homoiosmotic organisms, are unable to maintain an osmotic pressure lower than that in the external environment. Poikiloosmic organisms can be stenohaline or euryhaline. In euryhalynnas, the internal osmotic pressure varies widely in parallel with changes in salinity in the external environment. Some organisms (e.g., amphipod crustaceans) are homoeosmotic at low salinity within the range of salinity changes they can tolerate, but become poikilosmotic at high salinity.

Poikilothermy is a special form of physiological-ecological adaptation that provides invertebrates, fish, and reptiles with the ability to regulate their body temperature depending on the temperature of the external environment.