The Collaborative International Dictionary of English v.0.48:

Allegiance \Al*le"giance\, n. [OE. alegeaunce; pref. a- + OF.
   lige, liege. The meaning was influenced by L. ligare to bind,
   and even by lex, legis, law. See Liege, Ligeance.]
   1. The tie or obligation, implied or expressed, which a
      subject owes to his sovereign or government; the duty of
      fidelity to one's king, government, or state.
      [1913 Webster]

   2. Devotion; loyalty; as, allegiance to science.
      [1913 Webster]

   Syn: Loyalty; fealty.

   Usage: Allegiance, Loyalty. These words agree in
          expressing the general idea of fidelity and attachment
          to the "powers that be." Allegiance is an obligation
          to a ruling power. Loyalty is a feeling or sentiment
          towards such power. Allegiance may exist under any
          form of government, and, in a republic, we generally
          speak of allegiance to the government, to the state,
          etc. In well conducted monarchies, loyalty is a
          warm-hearted feeling of fidelity and obedience to the
          sovereign. It is personal in its nature; and hence we
          speak of the loyalty of a wife to her husband, not of
          her allegiance. In cases where we personify, loyalty
          is more commonly the word used; as, loyalty to the
          constitution; loyalty to the cause of virtue; loyalty
          to truth and religion, etc.
          [1913 Webster]

                Hear me, recreant, on thine allegiance hear me!
                                                  --Shak.
          [1913 Webster]

                So spake the Seraph Abdiel, faithful found, . .
                .
                Unshaken, unseduced, unterrified,
                His loyalty he kept, his love, his zeal.
                                                  --Milton.
          [1913 Webster]

WordNet (r) 3.0 (2006):

allegiance
    n 1: the act of binding yourself (intellectually or emotionally)
         to a course of action; "his long commitment to public
         service"; "they felt no loyalty to a losing team" [syn:
         commitment, allegiance, loyalty, dedication]
    2: the loyalty that citizens owe to their country (or subjects
       to their sovereign) [syn: allegiance, fealty]

The Devil's Dictionary (1881-1906):

ALLEGIANCE, n.

    This thing Allegiance, as I suppose,
    Is a ring fitted in the subject's nose,
    Whereby that organ is kept rightly pointed
    To smell the sweetness of the Lord's anointed.
                                                                  G.J.