Desmethyl is a term used in organic chemistry to refer to a methyl group that has been removed. It is often substituted in nomenclature for the more general Nor- prefix, the removal of one carbon atom along with the accompanying hydrogen atoms, however this prefix can be used to refer to two other structure changes.
Examples include desmethyltramadol and desmethylsertraline.
References
Desmethyl Wikipedia(Text) CC BY-SA