Animal glossary

This glossary defines 53 terms you will meet in animal profiles and conservation news, in plain English. Each definition is one or two sentences, and where we have a fuller guide, the term links to it. The IUCN categories use the wording of the IUCN Red List Categories and Criteria.

Updated 22 October 2026

Classifying animals

Taxonomy: The science of naming, describing and grouping living things. Animals are ranked from kingdom down through phylum, class, order, family and genus to species. Browse by class
Species: The basic unit of classification. A common working definition is a group of organisms that can interbreed in nature and produce fertile offspring, although biologists use several definitions.
Binomial name also: scientific name: The two-part Latin name given to every species: the genus followed by the species, written in italics, such as Panthera leo for the lion. Lion profile
Vertebrate: An animal with a backbone. Mammals, birds, reptiles, amphibians and fish are vertebrates. Vertebrates explained
Invertebrate: An animal without a backbone. Invertebrates make up the great majority of animal species and include insects, spiders, molluscs, worms and jellyfish. Invertebrates explained
Arthropod: An invertebrate with a hard exoskeleton, a segmented body and jointed legs. Insects, spiders, crustaceans and centipedes are arthropods. Arthropods explained
Mammal: A vertebrate that has hair or fur and feeds its young on milk from mammary glands. Most give birth to live young. What is a mammal?
Marsupial: A mammal whose young are born at a very early stage and usually finish developing in a pouch, such as kangaroos, koalas and opossums. Marsupials vs placentals
Monotreme: One of the few mammals that lay eggs: the platypus and the echidnas.
Bird: A warm-blooded vertebrate with feathers, a toothless beak and hard-shelled eggs. All living birds have wings, though some cannot fly. What is a bird?
Reptile: A vertebrate with dry, scaly skin that breathes air and mostly lays eggs with a leathery or hard shell. Snakes, lizards, turtles and crocodilians are reptiles. What is a reptile?
Amphibian: A vertebrate with thin, moist skin that usually starts life in water with gills and later breathes air. Frogs, toads, salamanders and newts are amphibians. What is an amphibian?
Fish: A cold-blooded vertebrate that lives in water, breathes through gills and usually has fins. Sharks and rays are fish with skeletons of cartilage rather than bone. What is a fish?

Bodies and life cycles

Endotherm also: warm-blooded: An animal that keeps its body temperature steady using heat from its own metabolism. Mammals and birds are endotherms. Warm-blooded vs cold-blooded
Ectotherm also: cold-blooded: An animal whose body temperature depends mainly on its surroundings, so it basks or seeks shade to warm up or cool down. Reptiles, amphibians and most fish are ectotherms. Warm-blooded vs cold-blooded
Metamorphosis: A major change in body form as an animal develops, such as a tadpole becoming a frog or a caterpillar becoming a butterfly. Metamorphosis explained
Gestation: The time between conception and birth in animals that give birth to live young. It ranges from about two weeks in some opossums to around 22 months in elephants.
Oviparous: Laying eggs that hatch outside the mother's body, as birds, most reptiles and most fish do.
Viviparous: Giving birth to live young that developed inside the mother's body, as most mammals and some sharks, snakes and lizards do.
Exoskeleton: A hard outer skeleton that supports and protects the body. Arthropods must shed (moult) it to grow.
Ruminant: A hoofed mammal with a stomach of several chambers that brings food back up to chew again (chewing the cud). Cattle, deer, giraffes and antelopes are ruminants.

Behaviour

Nocturnal: Active mainly at night and resting during the day, like owls, bats and many small mammals. Nocturnal animals explained
Diurnal: Active mainly during the day and resting at night.
Crepuscular: Active mainly at dawn and dusk. Many deer, rabbits and domestic cats are crepuscular.
Hibernation: A long period of deep inactivity in which body temperature, heart rate and breathing drop to save energy through winter. True hibernators include many bats, hedgehogs and ground squirrels. Hibernation explained
Brumation: A winter slowdown in reptiles, similar to hibernation, in which they become inactive but may wake to drink or move on warmer days.
Migration: A regular, usually seasonal journey between areas, often between breeding and feeding grounds. Why birds migrate
Echolocation: Finding objects by making sounds and listening to the echoes. Most bats and toothed whales such as dolphins use it to navigate and hunt. How bats use echolocation
Camouflage: Colours, patterns or shapes that help an animal blend into its surroundings, so predators or prey are less likely to see it. Animal camouflage explained
Mimicry: When one species evolves to resemble another, for example a harmless hoverfly that looks like a stinging wasp.
Bioluminescence: Light produced by a chemical reaction inside a living thing, as in fireflies, many deep-sea fish and some jellyfish. Bioluminescence explained
Venomous: Able to inject a toxin by a bite or sting. A poisonous animal, by contrast, is harmful when touched or eaten. Poison vs venom

Ecology and habitats

Habitat: The place where an animal naturally lives and finds food, shelter and mates. Habitats and biomes
Biome: A large region defined by its climate and the plants and animals adapted to it, such as tropical rainforest, desert, tundra or savanna. What is a biome?
Ecosystem: A community of living things together with the non-living environment they interact with, such as a coral reef or a pond.
Food web: The network of feeding relationships in an ecosystem. A food chain is one path through it, from plants to plant-eaters to predators. Food chains and food webs
Carnivore, herbivore and omnivore: An animal that eats mainly other animals, mainly plants, or both. Carnivore vs herbivore vs omnivore
Apex predator: A predator at the top of its food chain, with no natural predators as a healthy adult. Orcas, lions and saltwater crocodiles are examples. What is an apex predator?
Keystone species: A species with an effect on its ecosystem far larger than its numbers suggest. Sea otters, which keep sea urchins from destroying kelp forests, are a classic example. Keystone species explained
Symbiosis: A close, long-term relationship between two species. It can benefit both (mutualism), benefit one without affecting the other (commensalism), or benefit one at the other's cost (parasitism). Symbiotic relationships
Endemic: Found naturally in one place and nowhere else. Lemurs, for example, are endemic to Madagascar.
Biodiversity: The variety of life at every level: genes, species and ecosystems. What is biodiversity?

Conservation

IUCN Red List: The International Union for Conservation of Nature's global assessment of extinction risk for species, using published categories and criteria. A global category can differ from a species' status in one country. Understanding the IUCN Red List
Least Concern (LC): IUCN category for species assessed and found not to qualify for a threatened or Near Threatened category. Widespread and abundant species fall here.
Near Threatened (NT): IUCN category for species close to qualifying for a threatened category, or likely to qualify in the near future.
Vulnerable (VU): IUCN threatened category: the species faces a high risk of extinction in the wild.
Endangered (EN): IUCN threatened category: the species faces a very high risk of extinction in the wild. What makes an animal endangered?
Critically Endangered (CR): IUCN threatened category: the species faces an extremely high risk of extinction in the wild.
Extinct in the Wild (EW): IUCN category for species that survive only in captivity, in cultivation or as naturalised populations well outside their former range.
Data Deficient (DD): IUCN category used when there is not enough information to judge a species' extinction risk. It is not a sign that the species is safe.
CITES: The Convention on International Trade in Endangered Species of Wild Fauna and Flora, an agreement between governments that controls international trade in listed species through three appendices.
Habitat fragmentation: The breaking up of continuous habitat into smaller, isolated patches, often by roads, farms or towns. It can cut animals off from food, mates and migration routes. Why habitats disappear
Invasive species: A species moved by people outside its natural range that spreads and causes harm to wildlife, the economy or human health.

Sources

IUCN Red List Categories and Criteria (category definitions)
CITES: What is CITES?
Other definitions follow standard zoology usage and are explained, with references, in the linked guides. Report an error.

Questions about animal terms

What is the difference between Endangered and Vulnerable?

Both are IUCN threatened categories. Vulnerable means a high risk of extinction in the wild; Endangered means a very high risk. Critically Endangered is the highest threatened category.

Is cold-blooded the same as ectotherm?

Yes, in everyday use. Biologists prefer ectotherm, because a basking lizard's blood can be warmer than a mammal's. The point is where the heat comes from: the surroundings rather than the animal's own metabolism.

What is the difference between venomous and poisonous?

A venomous animal injects a toxin by biting or stinging, as snakes and spiders do. A poisonous animal is harmful when touched or eaten, as poison dart frogs are.