The Collaborative International Dictionary of English v.0.48:

Panama hat \Pan`a*ma" hat`\
   A fine plaited hat, made in Central America of the young
   leaves of a plant (Carludovica palmata).
   [1913 Webster]

WordNet (r) 3.0 (2006):

Panama hat
    n 1: a stiff hat made of straw with a flat crown [syn: boater,
         leghorn, Panama, Panama hat, sailor, skimmer,
         straw hat]