Dormancy is a state of reduced metabolic activity that many organisms adopt during periods of environmental stress. The stress may be expected (e.g., winter and summer, two common times of dormancy) or unexpected (e.g., drought). Dormancy during winter and summer is called hibernation and aestivation, respectively.
As noted above, hibernation is a behavior of certain animals, particularly in cold climates, of entering dormancy as a means of coping with the vicissitudes of winter. While many animals will sleep more than usual during the winter, true hibernation involves dramatically reduced heartrates and body temperatures, generally over a single unbroken period of torpor.
Most birds migrate to avoid harsh seasonal conditions; the common poorwill, a type of nightjar, is the only bird known to hibernate. Historically, swallows were believed to hibernate at the bottom of ponds.
As noted above, aestivation is a behavior of certain animals, typically in hot climates, involving a period of dormant behaviour to avoid the extremes of summer. It is a rather less common behaviour than hibernation. Animals that aestivate include certain types of crocodile and lungfishes.
The words "dormancy" (noun) and "dormant" (adjective) are also used more generally to describe things that are not active. For example, a dormant volcano is one that is not erupting, and the term "Dormant Commerce Clause" refers to situations in which Congress has not exercised its powers under the Commerce Clause.