The Collaborative International Dictionary of English v.0.48:

Agincourt \Agincourt\ n.
   1. a battle in which English longbowmen under Henry V
      decisively defeated a much larger French army in 1415. It
      was named for the site at which it occurred.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

Agincourt
    n 1: a battle in northern France in which English longbowmen
         under Henry V decisively defeated a much larger French army
         in 1415