The Collaborative International Dictionary of English v.0.48:

Inboard \In"board`\, a. & adv.
   1. (Naut.) Inside the line of a vessel's bulwarks or hull;
      the opposite of outboard; as, an inboard engine; an
      inboard cargo; haul the boom inboard.
      [1913 Webster]

   2. (Mech.) From without inward; toward the inside; as, the
      inboard stroke of a steam engine piston, the inward or
      return stroke.
      [1913 Webster]

WordNet (r) 3.0 (2006):

inboard
    adj 1: located within the hull or nearest the midline of a
           vessel or aircraft; "the inboard flaps on the wing" [ant:
           outboard]