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 ]