any of a class of compounds in which the four hydrogen atoms of the ammonium radical are replaced by other groups, generally organic radicals: used as solvents, disinfectants, etc.
See quaternary ammonium compound in American Heritage Dictionary 4
noun
Any of a group of compounds in which a central nitrogen atom is joined to four organic radicals and one acid radical, used as antiseptics, solvents, and emulsifying agents.