The Collaborative International Dictionary of English v.0.48:

Aboard \A*board"\, prep.
   1. On board of; as, to go aboard a ship.
      [1913 Webster]

   2. Across; athwart. [Obs.]
      [1913 Webster]

            Nor iron bands aboard
            The Pontic Sea by their huge navy cast. --Spenser.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Aboard \A*board"\, adv. [Pref. a- on, in + board.]
   1. On board; into or within a ship or boat; hence, into or
      within a railway car.
      [1913 Webster]

   2. Alongside; as, close aboard.
      [1913 Webster] (Naut.):

   To fall aboard of, to strike a ship's side; to fall foul
      of.

   To haul the tacks aboard, to set the courses.

   To keep the land aboard, to hug the shore.

   To lay (a ship) aboard, to place one's own ship close
      alongside of (a ship) for fighting.
      [1913 Webster]

WordNet (r) 3.0 (2006):

aboard
    adv 1: on a ship, train, plane or other vehicle [syn: aboard,
           on board]
    2: on first or second or third base; "Their second homer with
       Bob Allison aboard" [syn: aboard, on base]
    3: side by side; "anchored close aboard another ship" [syn:
       aboard, alongside]
    4: part of a group; "Bill's been aboard for three years now"

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "aboard":
   afloat, all aboard, aloft, among us, athwart the hawse,
   athwarthawse, aye, before the mast, here, hereabout, hereabouts,
   hereat, hereinto, hereto, hereunto, hither, hitherward,
   hitherwards, in sail, in this place, in this vicinity, just here,
   on board, on board ship, on deck, on shipboard, on the spot,
   somewhere about, to this place, topside, with us