on or into (a ship, aircraft, train, or other vehicle): [as adverb]:the plane crashed, killing all 158 people aboard figurativehe came aboard as IBM’s new chairman [as preposition]:I climbed aboard the yacht
on or on to (a horse): [as adverb]:with Richard Migliore aboard, he won the cup at a gallop
Baseball on base: [as adverb]:putting their first batter aboard
a call warning passengers to get on a ship, train, or bus that is about to depart.
late Middle English: from a-2 (expressing motion) + board, reinforced by Old French à bord