The Collaborative International Dictionary of English v.0.48:

Diplomacy \Di*plo"ma*cy\, n. [F. diplomatie. This word, like
   supremacy, retains the accent of its original. See
   Diploma.]
   1. The art and practice of conducting negotiations between
      nations (particularly in securing treaties), including the
      methods and forms usually employed.
      [1913 Webster]

   2. Dexterity or skill in securing advantages; tact.
      [1913 Webster]

   3. The body of ministers or envoys resident at a court; the
      diplomatic body. [R.] --Burke. Diplomat

WordNet (r) 3.0 (2006):

diplomacy
    n 1: negotiation between nations [syn: diplomacy, diplomatic
         negotiations]
    2: subtly skillful handling of a situation [syn: delicacy,
       diplomacy, discreetness, finesse]
    3: wisdom in the management of public affairs [syn:
       statesmanship, statecraft, diplomacy]

Moby Thesaurus II by Grady Ward, 1.0:

117 Moby Thesaurus words for "diplomacy":
   Eisenhower Doctrine, Machiavellianism, Machiavellism,
   Monroe Doctrine, Nixon Doctrine, Truman Doctrine, ability, address,
   adeptness, adroitness, airmanship, appeasement, artfulness,
   artisanship, artistry, balance of power, bravura, brilliance,
   brinkmanship, capability, capacity, cleverness, coexistence,
   colonialism, command, competence, compromise, containment, control,
   coordination, craft, craftsmanship, cunning, deftness, detente,
   deterrence, dexterity, dexterousness, dextrousness, diplomatic,
   diplomatics, discretion, dollar diplomacy, dollar imperialism,
   efficiency, expansionism, expertise, facility, finesse,
   foreign affairs, foreign policy, good-neighbor policy, grace, grip,
   handiness, horsemanship, imperialism, ingeniousness, ingenuity,
   internationalism, intrigue, isolationism, jobbery, jobbing,
   know-how, machination, maneuvering, manifest destiny, marksmanship,
   mastership, mastery, militarism, nationalism, negotiation,
   neocolonialism, neutralism, nonresistance, open door,
   open-door policy, peace offensive, peaceful coexistence, poise,
   politics, practical ability, preparedness, proficiency, prowess,
   quickness, readiness, resource, resourcefulness, savoir faire,
   savoir-faire, savvy, seamanship, shirt-sleeve diplomacy,
   shuttle diplomacy, skill, skillfulness, spheres of influence,
   statecraft, statesmanship, style, tact, tactfulness,
   technical brilliance, technical mastery, technical skill,
   technique, the big stick, timing, tough policy, virtuosity, wit,
   wizardry, workmanship, world politics



Bouvier's Law Dictionary, Revised 6th Ed (1856):

DIPLOMACY., The science which treats of the relations and interests of
nations with nations.



The Devil's Dictionary (1881-1906):

DIPLOMACY, n.  The patriotic art of lying for one's country.