<p><strong>Adverb</strong></p><ul><li>On board; on, in, or into a ship, train, airplane, bus, etc.: to step aboard.</li><li>Alongside; to the side.</li><li>Baseball: on base: a homer with two aboard.</li><li>Into a group as a new member: The office manager welcomed him aboard.</li></ul><p><strong>Preposition</strong></p><ul><li>On board of; on, in, or into: to come aboard a ship.</li></ul>