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

   Ordinary seaman. See Ordinary.
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The Collaborative International Dictionary of English v.0.48:

Ordinary \Or"di*na*ry\, a. [L. ordinarius, fr. ordo, ordinis,
   order: cf. F. ordinaire. See Order.]
   1. According to established order; methodical; settled;
      regular. "The ordinary forms of law." --Addison.
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   2. Common; customary; usual. --Shak.
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            Method is not less requisite in ordinary
            conversation that in writing.         --Addison.
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   3. Of common rank, quality, or ability; not distinguished by
      superior excellence or beauty; hence, not distinguished in
      any way; commonplace; inferior; of little merit; as, men
      of ordinary judgment; an ordinary book.
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            An ordinary lad would have acquired little or no
            useful knowledge in such a way.       --Macaulay.
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   Ordinary seaman (Naut.), one not expert or fully skilled,
      and hence ranking below an able seaman.
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   Syn: Normal; common; usual; customary.

   Usage: See Normal. -- Ordinary, Common. A thing is
          common in which many persons share or partake; as, a
          common practice. A thing is ordinary when it is apt to
          come round in the regular common order or succession
          of events.
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