In ethology, territory is the sociographical area that an animal consistently defends against conspecific competition (or, occasionally, against animals of other species) using agonistic behaviors or (less commonly) real physical aggression. Animals that actively defend territories in this way are referred to as being territorial or displaying territorialism.

Territoriality is only shown by a minority of species. More commonly, an individual or a group of animals occupies an area that it habitually uses but does not necessarily defend; this is called its home range. The home ranges of different groups of animals often overlap, and in these overlap areas the groups tend to avoid each other rather than seeking to confront and expel each other. Within the home range there may be a core area that no other individual group uses, but, again, this is as a result of avoidance.

The ultimate function of animals inhabiting and defending a territory is to increase the individual fitness or inclusive fitness of the animals expressing the behaviour. Fitness in this biological sense relates to the ability of an animal to survive and raise young. The proximate functions of territory defense vary. For some animals, the reason for such protective behaviour is to acquire and protect food sources, nesting sites, mating areas, or to attract a mate.

Among birds, territories have been classified as six types.

Reports of territory size can be confused by a lack of distinction between home range and the defended territory. The size and shape of a territory can vary according to its purpose, season, the amount and quality of resources it contains, or the geography. The size is usually a compromise of resource needs, defense costs, predation pressure and reproductive needs.

Some species of squirrels may claim as much as 10 hectares (25 acres) of territory. For European badgers, a home range may be as small as 30 hectares (74 acres) in a good rural habitat, but as large as 300 hectares (740 acres) in a poor habitat. On average, a territory may be approximately 50 hectares (120 acres), with main setts normally at least 500 metres (1,600 ft) apart. In urban areas, territories can be as small as 5 hectares (12 acres), if they can obtain enough food from bird tables, food waste or artificial feeding in suburban gardens. Spotted hyenas (Crocuta crocuta) have highly variable territory sizes, ranging from less than 4,000 hectares (9,900 acres) in the Ngorongoro Crater to over 100,000 hectares (250,000 acres) in the Kalahari.

In birds, golden eagles (Aquila chrysaetos) have territories of 9,000 hectares (22,000 acres), least flycatchers' (Empidonax minimus) territories are about 600 square metres (6,500 sq ft) and gulls have territories of only a few square centimetres in the immediate vicinity of the nest.

Territories can be linear. Sanderlings (Calidris alba) forage on beaches and sandflats. When on beaches, they feed either in flocks or individual territories of 10 to 120 metres of shoreline.