A toxin, in a scientific context, is a biologically produced substance that causes injury to the health of a living thing on contact or absorption, typically by interacting with biological macromolecules such as enzymes and receptors.

The term has also attained widespread use, inaccurately, in the context of complementary medicine and quackery, where it refers to generic harmful substances (often of unspecified and/or unprovable chemistry) claimed to cause ill-health.

Many plants, animals and microorganisms generate natural toxins to discourage or kill predators. Animal toxins that are delivered subcutaneously (e.g. by sting or bite) are also called venom. In normal usage, a poisonous organism is one that is harmful to consume, but a venomous organism uses poison to defend itself while still alive. A single organism can be both venomous and poisonous.

Toxins are also generated by bacteria, either in the living body during infection (for instance, by tetanus) or in decaying biological material. Exotoxins are secreted externally by the bacteria; Endotoxin forms part of the cell wall. Food poisoning is a term for the broad range of illnesses that can result from eating food that is spoiled or tainted by bacterial toxins, such as Endotoxin, botulinum, and the so-called Shiga-like toxin secreted by the emergent E. coli strain E. coli O157:H7.

Naturally occurring or human-modified toxins may be intentionally released by humans in chemical warfare.