The ancient Greek εὐρύ is wide and the ancient Greek ἅλς is salt.

Euryhaline fishes are fishes that are able to withstand large fluctuations in water salinity. They include many inhabitants of river mouths, brackish and ultra-haline reservoirs and passerine fish.

An example of a euryhaline fish is Poecilia sphenops, which can live in fresh, brackish or salt water.

Carcinus maenas is an example of an euryhaline invertebrate that can live in saline and brackish water. Euryhaline organisms are commonly found in habitats such as estuaries and tidal basins where salinity changes regularly. However, some organisms are euryhaline because their life cycle involves migration between freshwater and marine environments, such as salmon and eels.

The opposite of euryhaline organisms are stenohaline organisms, which can only survive within a narrow range of salinity. Most freshwater organisms are stenohaline and die in seawater, and similarly most marine organisms are stenohaline and cannot live in fresh water.