aboard. 1  On board of; onto or into a ship, boat, train, plane. [First attested around 1350 to 1470.] 2012 March 1, William E. Carter, Merri Sue Carter, âThe British Longitude Act Reconsideredâ, in American Scientist, volume 100, number 2, page 87: Conditions were horrendous aboard most British naval vessels at the time. 2  Onto a horse.