The Collaborative International Dictionary of English v.0.48:

Customhouse \Cus"tom*house"\ (-hous`), n.
   The building where customs and duties are paid, and where
   vessels are entered or cleared.
   [1913 Webster]

   Customhouse broker, an agent who acts for merchants in the
      business of entering and clearing goods and vessels.
      [1913 Webster]

WordNet (r) 3.0 (2006):

customhouse
    n 1: a government building where customs are collected and where
         ships are cleared to enter or leave the country [syn:
         customhouse, customshouse]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

CUSTOM-HOUSE. A place appointed by law, in ports of entry, where importers
of goods, wares and merchandise are bound to enter the same, in order to pay
or secure the duties or customs due to the government.