dioxin

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of several carcinogenic, mutagenic, and teratogenic polychlorinated heterocyclic aromatic hydrocarbons that can occur as impurities in petroleum-derived herbicides and as byproducts of manufacturing chemicals and burning fuels and waste.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun organic chemistry any of a broad range of toxic or carcinogenic halogenated polycyclic compounds that occur as byproducts of herbicides
• noun organic chemistry the parent compound, dibenzo-p-dioxin, in which two benzene rings are connected vio two oxygen atoms; oxanthrene
• noun organic chemistry the unsaturated six-membered heterocycle having four carbon atoms, two oxygen atoms and two double bonds

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any of several toxic or carcinogenic hydrocarbons that occur as impurities in herbicides