Venom or zootoxin (literally, animal poison) is any of a variety of toxins used by several groups of animal species, for the purpose of defense and hunting prey. Though subjective, the definition of a venom differs from a poison (as used in most zoology and medicine texts). Generally, a venom is defined as a biologic toxin that is injected to cause its effect while a poison is a biologic toxin which is absorbed through epithelial linings (either of the gut or through the skin).

The animals most widely known to use venom are snakes, some species of which inject venom into their prey through hollow fangs; spiders and centipedes, which also inject venom through fangs; scorpions and stinging insects, which inject venom with a sting (which is a modified egg-laying device - the ovipositor). There are also many caterpillars that have defensive venom glands associated with specialized bristles on the body, known as urticating hairs, some of which can be lethal to humans (e.g., the Lonomia moth). Venom is also found in other reptiles besides snakes such as the gila monster, and mexican beaded lizard. Other insects, such as true bugs *, also produce venom. However, venom can also be found in some fish, such as the cartilaginous fishes: stingrays, sharks, and chimaeras and the teleost fishes, which include: monognathus eels, catfishes, stonefishes and waspfishes, scorpionfishes and lionfishes, gurnard perches, rabbitfishes, surgeonfishes, scats, stargazers, weevers, carangids, saber-toothed blenny, and toadfish. In fact, recent studies have shown that there are more venomous ray-finned fishes than all other venomous vertebrates combined. Additionally, there are many other venomous animals, including jellyfishes, cone snails, bees, wasps, ants, solenodons, shrews, the slow loris, and the male platypus.

Bees use an acidic venom (apitoxin) designed to cause pain to the stung, because their purpose is to defend their home and food stores, while wasps use a chemically different venom designed to paralyze the prey, so it can be stored alive in the food chambers of their young. The use of venom is much more widespread than just these examples, of course.

It is important to note the difference between "venomous" and "poisonous", which are two commonly confused terms with regards to plant and animal life. Venomous, as stated above, refers to animals who venom into their prey or as a self-defence mechanism while the organism is still alive. Poisonous, on the other hand, refers to plants or animals that are harmful when consumed or touched. One bird species the hooded pitohui, although not venomous, is poisonous, secreting a neurotoxin on to its skin and feathers. The slow loris, a primate, blurs the boundary between poisonous and venomous; it has poison secreting patches on the inside of its elbows which it is believed to smear on its young to prevent them from being eaten. However, it will also lick these patches, giving it a venomous bite.