- A poisonous substance, especially a protein, that is produced by living cells or organisms and is capable of causing disease when introduced into the body tissues but is often also capable of inducing neutralizing antibodies or antitoxins.
- A poisonous or harmful nonbiological substance, such as a pollutant.
- Any toxic ptomaine.
- A specific poison of albuminous character, immunization with which leads to the production of a specific antitoxin.
- A poisonous product formed by an organism, such as a pathogenic bacterium, a plant or an animal, usually having a high molecular weight, often a protein or a polysaccharide, but occasionally a low-molecular weight agent such as <stype>tetrodotoxin</stype>.
- A <xref>toxic</xref> or <xref>poisonous</xref> substance produced by the biological processes of biological organisms.
- a poisonous substance produced during the metabolism and growth of certain microorganisms and some higher plant and animal species