Core temperature is the operating temperature of an organism, specifically in deep structures of the body such as the liver, in comparison to temperatures of peripheral tissues. Most physicians believe temperature taken in the rectum is closest to core temperature. Temperature is part of a homeostatic mechanism designed to keep the organism at optimum operating temperature. Most of the time these temperatures allow enzymes to work in chemical reactions.

In humans this optimum temperature is 98.6 Fahrenheit or 37 Celsius degrees.