From the Latin tolerantia patience.

Tolerant species are hydrobionts that are able to withstand deviations of environmental factors from their optimal values. Organisms with a wide range of tolerance are designated by the prefix "eury", while organisms with a narrow range of tolerance have the prefix "steno" added.

The range between the environmental minimum and maximum of a factor constitutes the tolerance limit. Tolerant organisms are organisms that are resistant to unfavorable environmental changes.