The Collaborative International Dictionary of English v.0.48:

Seaman \Sea"man\, n.; pl. Seamen.
   A merman; the male of the mermaid. [R.] "Not to mention
   mermaids or seamen." --Locke.
   [1913 Webster]

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):

seaman
    n 1: a man who serves as a sailor [syn: mariner, seaman,
         tar, Jack-tar, Jack, old salt, seafarer, gob,
         sea dog]
    2: muckraking United States journalist who exposed bad
       conditions in mental institutions (1867-1922) [syn: Seaman,
       Elizabeth Seaman, Elizabeth Cochrane Seaman, Nellie
       Bly]