Psychrophiles (=cold-loving) are extremophilic bacteria or archaea that have an optimal growth-temperature at or below 15°C, and a maximal growth-temperature at about 20°C. Their lower limit for growth is at or below 0°C.

Psychrotrophs (=cold-tolerant) on the other hand are organisms that can grow at low temperatures, but have optimal growth temperatures above 15 °C and max. growth temperatures above 20°C.