Apparent death is a behavior in which animals take on the appearance of being dead. It is an immobile state most often triggered by a predatory attack and can be found in a wide range of animals from insects and crustaceans to mammals, birds, reptiles, amphibians, and fish. Apparent death is separate from the freezing behavior seen in some animals.

Apparent death is a form of animal deception considered to be an anti-predator strategy, but it can also be used as a form of aggressive mimicry. When induced by humans, the state is sometimes colloquially known as animal hypnosis. The earliest written record of "animal hypnosis" dates back to the year 1646 in a report by Athanasius Kircher, in which he subdued chickens.

Tonic immobility (also known as the act of feigning death, or exhibiting thanatosis) is a behaviour in which some animals become apparently temporarily paralysed and unresponsive to external stimuli. Tonic immobility is most generally considered to be an anti-predator behavior because it occurs most often in response to an extreme threat such as being captured by a (perceived) predator. Some animals use it to attract prey or facilitate reproduction. For example, in sharks exhibiting the behaviour, some scientists relate it to mating, arguing that biting by the male immobilizes the female and thus facilitates mating.

Despite appearances, some animals remain conscious throughout tonic immobility. Evidence for this includes the occasional responsive movement, scanning of the environment and animals in tonic immobility often taking advantage of escape opportunities. Tonic immobility is preferred in the literature because it has neutral connotations compared to 'thanatosis' which has a strong association with death.

Tonic immobility is different from freezing behavior in animals. A deer in headlights and an opossum "playing dead" are common examples of an animal freezing and tonic immobility, respectively. Freezing occurs early during a predator-prey interaction when the prey detects and identifies the threat, but the predator has not yet seen the prey. Because freezing occurs before detection and is used to better camouflage the prey and prevent the predator from attacking, it is considered a primary defense mechanism.

Tonic immobility occurs after the predator has detected and or made contact with the prey, and is likely used to prevent further attack by the predator or consumption of the prey. Because tonic immobility occurs later in the predator attack sequence, it is considered a secondary defense mechanism and is therefore distinct from freezing. Although freezing animals become rigid, they often stay upright and do not change their posture while frozen whereas during tonic immobility, animals often become rigid and change their posture.

Freezing behavior and tonic immobility are similar in that both may induce bradycardia (slowing of the heart rate), but the freezing response may instead be accompanied by rapid or increased breathing rate, increased heart rate, increased blood pressure and inhibition of digestion, depending on whether the sympathetic or parasympathetic nervous system is engaged. In contrast, along with bradycardia, vertebrates in tonic immobility often reduce their breathing rate or protrude their tongue, further distinguishing this behavior from the freezing response.

For defensive purposes, thanatosis hinges on the pursuer's becoming unresponsive to its victim, as most predators only catch live prey.