Source : Webster's Revised Unabridged Dictionary (1913)

Derby \Der"by\ (?; usually ? in Eng.; 85), n.
   1. A race for three-old horses, run annually at Epsom (near
      London), for the Derby stakes. It was instituted by the
      12th Earl of Derby, in 1780.

   {Derby Day}, the day of the annual race for the Derby stakes,
      -- Wednesday of the week before Whitsuntide.

   2. A stiff felt hat with a dome-shaped crown.