The Collaborative International Dictionary of English v.0.48:

customs \customs\ n. pl.
   1. money collected under a tariff; a duty imposed on imported
      goods.

   Syn: customs duty, custom, impost.
        [WordNet 1.5]

   2. the government department administering the collection of
      import tariffs. Used with as singular verb.
      [PJC]

   3. that area within an airport, sea port, or other border
      station where freight or the baggage of travellers is
      checked for dutiable materials or contraband; as, it took
      an hour to get through customs.
      [PJC]

WordNet (r) 3.0 (2006):

customs
    n 1: money collected under a tariff [syn: customs, customs
         duty, custom, impost]