The Collaborative International Dictionary of English v.0.48:

Yankee \Yan"kee\, n. [Commonly considered to be a corrupt
   pronunciation of the word English, or of the French word
   Anglais, by the native Indians of America. According to
   Thierry, a corruption of Jankin, a diminutive of John, and a
   nickname given to the English colonists of Connecticut by the
   Dutch settlers of New York. Dr. W. Gordon ("Hist. of the
   Amer. War," ed, 1789, vol. i., pp. 324, 325) says it was a
   favorite cant word in Cambridge, Mass., as early as 1713, and
   that it meant excellent; as, a yankee good horse, yankee good
   cider, etc. Cf. Scot yankie a sharp, clever, and rather bold
   woman, and Prov. E. bow-yankees a kind of leggins worn by
   agricultural laborers.]
   A nickname for a native or citizen of New England, especially
   one descended from old New England stock; by extension, an
   inhabitant of the Northern States as distinguished from a
   Southerner; also, applied sometimes by foreigners to any
   inhabitant of the United States.
   [1913 Webster]

         From meanness first this Portsmouth Yankey rose,
         And still to meanness all his conduct flows.
                                                  --Oppression,
                                                  A poem by an
                                                  American
                                                  (Boston,
                                                  1765).
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Yankee \Yan"kee\, a.
   Of or pertaining to a Yankee; characteristic of the Yankees.
   [1913 Webster]

         The alertness of the Yankee aspect.      --Hawthorne.
   [1913 Webster]

   Yankee clover. (Bot.) See Japan clover, under Japan.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Yankee
    adj 1: used by Southerners for an inhabitant of a northern state
           in the United States (especially a Union soldier)
    n 1: an American who lives in the North (especially during the
         American Civil War) [syn: Yankee, Yank, Northerner]
    2: an American who lives in New England [syn: New Englander,
       Yankee]
    3: an American (especially to non-Americans) [syn: Yankee,
       Yank, Yankee-Doodle]

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "Yankee":
   Acadian, Anglo-Indian, Bolshevik, Bolshevist, Bolshie, Brooklynese,
   Cajun, Canadian French, Carbonarist, Carbonaro, Castroist,
   Castroite, Charley, Cockney, Communist, Cong, Down-Easter Yankee,
   Easterner, Fenian, French Canadian, Guevarist, Gullah, Jacobin,
   Leninist, Maoist, Marxist, Mau-Mau, Midland, Midland dialect,
   New England dialect, New Englander, Northener, Northman,
   Pennsylvania Dutch, Puritan, Red, Red Republican, Roundhead,
   Sinn Feiner, Trotskyist, Trotskyite, VC, Vietcong, Westerner,
   Yankee Doodle, Yorkshire, anarch, anarchist, bonnet rouge,
   bundle of isoglosses, class dialect, criminal syndicalist, dialect,
   dialect atlas, dialect dictionary, eastlander, idiom, isogloss,
   linguistic atlas, linguistic community, linguistic island,
   local dialect, localism, northlander, patois, provincialism, rebel,
   red, regional accent, regionalism, revolutionary,
   revolutionary junta, revolutioner, revolutionist, revolutionizer,
   sans-culotte, sans-culottist, southlander, speech community,
   subdialect, subversive, syndicalist, terrorist, westlander



The Devil's Dictionary (1881-1906):

YANKEE, n.  In Europe, an American.  In the Northern States of our
Union, a New Englander.  In the Southern States the word is unknown.
(See DAMNYANK.)