1. One of a group of organic heterocyclic compounds containing a diunsaturated ring of four carbon atoms and one oxygen atom. 2. C₄H₄O₄ The simplest furan type of molecule; a colorless, mildly toxic liquid, boiling at 32_C, insoluble in water, soluble in alcohol and ether; used as a chemical intermediate. Also known as furfuran; tetrol.