A toxin is a 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 is usually reserved for naturally produced substances that kill rapidly in small quantities, such as the bacterial proteins that cause tetanus and botulism. The word "toxic" is used more loosely and often applied to non-biological materials, as in "toxic waste" and "toxicology."

Ingestable toxins are also often referred to as poisons, especially when intentionally administered by a human. 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.

In most cases the quantity of material is related to its toxicity. Even a material such as water, which is normally considered non-toxic, can be toxic when ingested in sufficient quantity. Toxicity is measured in terms of the amount of the particular material that is needed to kill half the organisms in the test, which is called the LD50.

Some dietary minerals are actually nutrients for plants, animals, or humans at minute levels, but become toxic when the quantity is larger. Many plants, animals and microorganisms generate toxins to discourage or kill predators. Food poisoning is a term for a 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.