The Collaborative International Dictionary of English v.0.48:

Hansom \Han"som\ (h[a^]n"s[u^]m), n., Hansom cab \Han"som cab`\
   (k[a^]b`). [From the name of the inventor.]
   A light, low, two-wheeled covered carriage with the driver's
   seat elevated behind, the reins being passed over the top.
   [1913 Webster]

         He hailed a cruising hansom . . . " 'Tis the gondola of
         London," said Lothair.                   --Beaconsfield.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hansom
    n 1: a two-wheeled horse-drawn covered carriage with the
         driver's seat above and behind the passengers [syn:
         hansom, hansom cab]