From Greek στενός 'narrow' and Greek βίον 'living'.
Stenobionts are hydrobionts capable of tolerating slight variations in environmental factors (pH, oxygen, pressure, temperature, salinity, etc.) and occupy a narrow range, unlike eurybionts.
For example, one of the deepest fish, Pseudoliparis amblystomopsis, is known only from depths of 6-7 km, where it lives in the complete absence of light, a hydrostatic pressure of 600-700 am, at a constant low temperature and constant salinity.
Stenobionts are opposed to eurybionts. They can exist in a wide variety of conditions.