Pronunciation: (u-bôrd', u-bōrd'),[key] —adv. 1. on board; on, in, or into a ship, train, airplane, bus, etc.: to step aboard. 2. alongside; to the side.
3. Baseball.on base: a homer with two aboard. 4. all aboard!(as a warning to passengers entering or planning to enter a train, bus, boat, etc., just before starting) Everyone get on!
5. into a group as a new member: The office manager welcomed him aboard.
—prep. on board of; on, in, or into: to come aboard a ship.