An organism that lives in environmental conditions beyond normal conditions: There are microbes that thrive in situations that would kill other creatures, yet these extremophiles make their habitats in such forbidding environments as boiling or near freezing water, vinegar-like liquids, household ammonia, or in concentrated brine.

These microbes are called extremophiles because they thrive in environments that, from the human point of view, are clearly the farthest from normal situations and scientists have recognized that places once assumed to be sterile abound with a great deal of microbial life.