The Collaborative International Dictionary of English v.0.48:

Armada \Ar*ma"da\ ([aum]r*m[=a]"d[.a] or [aum]r*m[aum]"d[.a]),
   n. [Sp. armada, L. as if armata (sc. classic fleet), fr.
   armatus, p. p. of armare. See Arm, v. t. Army.]
   A fleet of armed ships; a squadron. Specifically, the Spanish
   fleet which was sent to assail England, a. d. 1558.
   [1913 Webster]

WordNet (r) 3.0 (2006):

armada
    n 1: a large fleet