The Collaborative International Dictionary of English v.0.48:

Tyrant \Tyr"ant\ (t[imac]"rant), n. [OE. tirant, tiraunt,
   tyraunt, OF. tiran, tirant (probably from confusion with the
   p. pr. of verbs), F. tyran, L. tyrannus, Gr. ty`rannos,
   originally, an absolute sovereign, but afterwards, a severe
   or cruel ruler.]
   1. An absolute ruler; a sovereign unrestrained by law or
      constitution; a usurper of sovereignty.
      [1913 Webster]

   Note: Free governments [in Greece] having superseded the old
         hereditary sovereignties (basilei^ai), all who obtained
         absolute power in a state were called ty`rannoi,
         tyrants, or rather despots; -- for the term rather
         regards the irregular way in which the power was
         gained, whether force or fraud, than the way in which
         it was exercised, being applied to the mild
         Pisistratus, but not to the despotic kings of Persia.
         However, the word soon came to imply reproach, and was
         then used like our tyrant. --Liddell & Scott.
         [1913 Webster]

   2. Specifically, a monarch, or other ruler or master, who
      uses power to oppress his subjects; a person who exercises
      unlawful authority, or lawful authority in an unlawful
      manner; one who by taxation, injustice, or cruel
      punishment, or the demand of unreasonable services,
      imposes burdens and hardships on those under his control,
      which law and humanity do not authorize, or which the
      purposes of government do not require; a cruel master; an
      oppressor. "This false tyrant, this Nero." --Chaucer.
      [1913 Webster]

            Love, to a yielding heart, is a king, but to a
            resisting, is a tyrant.               --Sir P.
                                                  Sidney.
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   3. (Zool.) Any one of numerous species of American
      clamatorial birds belonging to the family Tyrannidae; --
      called also tyrant bird.
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   Note: These birds are noted for their irritability and
         pugnacity, and for the courage with which they attack
         rapacious birds far exceeding them in size and
         strength. They are mostly plain-colored birds, but
         often have a bright-colored crown patch. A few species,
         as the scissorstail, are handsomely colored. The
         kingbird and pewee are familiar examples.
         [1913 Webster]

   Tyrant flycatcher (Zool.), any one of numerous species of
      tyrants which have a flattened bill, toothed at the tip,
      and resemble the true flycatchers in habits. The Acadian
      flycatcher (Empidonax Acadicus) and the vermilion
      flycatcher (Pyrocephalus rubineus) are examples.

   Tyrant shrike (Zool.), any one of numerous species of
      American tyrants of the genus Tyrannus having a strong
      toothed bill and resembling the shrikes in habits. The
      kingbird is an example.
      [1913 Webster]

WordNet (r) 3.0 (2006):

tyrant bird
    n 1: large American birds that characteristically catch insects
         on the wing [syn: New World flycatcher, flycatcher,
         tyrant flycatcher, tyrant bird]