The term hibernation is commonly applied to all types of winter dormancy in vertebrate animals. Thus defined, hibernators include many fishes, amphibians, and reptiles that overwinter with body temperatures near freezing, as well as bears and a few other mammals that spend most of the winter sleeping in dens. The latter, however, do not undergo much lowering of body temperature and are rather easily awakened; they are not considered true hibernators.

The true hibernator spends most of the winter in a state close to death; in fact, the animal may appear to be dead. The body temperature is close to 0° C (32° F); the respiration is only a few breaths per minute; and the heartbeat is so slow and gradual as to be barely perceptible. Exposed to moderate warmth, the animal slowly awakens, requiring an hour or more to reach an alert state.

Among mammals, true hibernators are found only in the orders Chiroptera (bats), Insectivora (hedgehogs and allies), and Rodentia (ground squirrels, marmots, etc.). Typically, the hibernator relies on a combination of reserve body fat, stored food supplies (in rodents only), and a protected den to enable it to survive the winter. At intervals of several weeks the animal elevates its body temperature, awakens, moves about, feeds, and then returns to its state of torpor.

In some desert regions, certain animals escape the rigours of summer drought by entering a torpid state, estivation, that is similar in many ways to hibernation.