A homeotherm is an organism whose body temperature is kept essentially constant under normal circumstances. The opposite is a poikilotherm - an organism whose body temperature varies widely. Homeothermy is a part of homeostasis.
For example, the body temperature of a mammal is a nearly constant - homeothermic - as opposed to a lizard, whose temperature can change considerably when it is basking in the sun or under water. A human will occasionally get a fever, but even then the body temperature only varies by about 2-3 °C and the basic body temperature stays nearly the same.