any of a family of organic chemical compounds, including chloroform, that form as when water supplies are chlorinated
See trihalomethane in American Heritage Dictionary 4
(trīˌhăl-ə-mĕthˈānˌ)
noun
A chemical compound containing three halogen atoms substituted for the three hydrogen atoms normally present in a methane molecule. It can occur in chlorinated water as a result of reaction between organic materials in the water and chlorine added as a disinfectant.