The Collaborative International Dictionary of English v.0.48:

Nationalism \Na"tion*al*ism\, n.
   1. The state of being national; national attachment;
      nationality.
      [1913 Webster]

   2. An idiom, trait, or character peculiar to any nation.
      [1913 Webster]

   3. National independence; the principles of the Nationalists.
      [1913 Webster]

   4. A devotion to one's country; patriotism; especially. an
      exaggerated or excessive form of patriotism; chauvinism.
      [PJC]

   5. The policy of advocating the independence of one's
      country.
      [PJC]

   6. The policy of advocating the interests of one's own
      country exclusively, regardless of effects of a country's
      actions on other countries.
      [PJC]

WordNet (r) 3.0 (2006):

nationalism
    n 1: love of country and willingness to sacrifice for it; "they
         rode the same wave of popular patriotism"; "British
         nationalism was in the air and patriotic sentiments ran
         high" [syn: patriotism, nationalism]
    2: the doctrine that your national culture and interests are
       superior to any other [ant: internationalism,
       multiculturalism]
    3: the aspiration for national independence felt by people under
       foreign domination
    4: the doctrine that nations should act independently (rather
       than collectively) to attain their goals [ant:
       internationalism]