The Collaborative International Dictionary of English v.0.48:

Etiquette \Et"i*quette`\, n. [F. prop., a little piece of paper,
   or a mark or title, affixed to a bag or bundle, expressing
   its contents, a label, ticket, OF.estiquete, of German
   origin; cf. LG. stikke peg, pin, tack, stikken to stick, G.
   stecken. See Stick, and cf. Ticket.]
   The forms required by good breeding, or prescribed by
   authority, to be observed in social or official life;
   observance of the proprieties of rank and occasion;
   conventional decorum; ceremonial code of polite society.
   [1913 Webster]

         The pompous etiquette to the court of Louis the
         Fourteenth.                              --Prescott.
   [1913 Webster]

WordNet (r) 3.0 (2006):

etiquette
    n 1: rules governing socially acceptable behavior

Moby Thesaurus II by Grady Ward, 1.0:

91 Moby Thesaurus words for "etiquette":
   amenities, approbatory, approving, behavior, bienseance, bon ton,
   civilities, civility, comity, commendatory, complimentary, conduct,
   conformity, consuetude, convenance, convention, conventional usage,
   conventionalism, conventionality, correct deportment, correctness,
   courtesy, courtliness, custom, decencies, decency, decorousness,
   decorum, deportment, dignity, diplomatic code, discourtesy,
   elegance, elegancies, encomiastic, established way, eulogistic,
   exquisite manners, fashion, folkway, form, formalities, formality,
   good behavior, good citizenship, good form, good manners,
   laudatory, manner, manners, misbehavior, mores, natural politeness,
   observance, panegyrical, point of etiquette, politeness, politesse,
   poor behavior, practice, praiseful, praxis, prescription,
   proper thing, proprieties, propriety, protocol, punctilio,
   quiet good manners, ritual, rules of conduct, sanctioned behavior,
   seemliness, sociability, social code, social conduct,
   social convention, social graces, social procedures, social usage,
   standard behavior, standard usage, standing custom,
   time-honored practice, tradition, usage, use, way, what is done,
   wont, wonting