The Collaborative International Dictionary of English v.0.48:

Seaman \Sea"man\, n.; pl. Seamen. [AS. saeman.]
   One whose occupation is to assist in the management of ships
   at sea; a mariner; a sailor; -- applied both to officers and
   common mariners, but especially to the latter. Opposed to
   landman, or landsman.
   [1913 Webster]

   Able seaman, a sailor who is practically conversant with
      all the duties of common seamanship.

   Ordinary seaman. See Ordinary.
      [1913 Webster]

WordNet (r) 3.0 (2006):

able seaman
    n 1: a seaman in the merchant marine; trained in special skills
         [syn: able seaman, able-bodied seaman]