What is the definition or description of: toxin?

Poisonous substance : Poisonous substance produced within living cells or organisms;man-made substances created by artificial processes are thus excluded. The term was first used by organic chemist ludwig brieger (1849–1919). For a toxic substance not produced within living organisms, "toxicant" and "toxics" are also sometimes used.[citation needed]. Toxins can be small molecules, peptides, or proteins.