From AMS Glossary
A thermal belt, either of latitude or altitude, in which the plant and animal life is of a distinctive type.
The limits of each life zone are mainly fixed by minimum temperatures. The major life zones are boreal, temperate, subtropical, and tropical in both Northern and Southern Hemispheres, with tropical being equatorial. Within each thermal life zone, moisture gradients provide further subdivisions into a number of biogeographical provinces. Latitudinal and altitudinal life zones are not strictly equivalent.