a·board / əˈbôrd/ • adv. & prep. on or into (a ship, aircraft, train, or other vehicle): [as adv.] welcome aboard, sir [as prep.] climbing aboard the yacht. ∎ fig. into an organization or team as a new member: [as adv.] coming aboard as IBM's new chairman. ∎ Baseball on base as a runner: putting their first batter aboard.PHRASES: all aboard! a call warning passengers to get on a ship, train, or bus that is about to depart.