quaternary ammonium compound

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. 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.

Wiktionary

1. n. chemistry An amine derivative, containing four radicals, of general formula R1R2R3R4-N+X-; the radicals may be the same, different, or part of a ring; X is any univalent anion such as OH-.

WordNet 3.0

1. n. a compound derived from ammonium with hydrogen atoms replaced by organic groups; used as surface-active agents, disinfectants, and in drugs