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.

Source: Webster's Revised Unabridged Dictionary (1913)