Aboard adv. : On board; into or within a ship or boat; hence, into or within a railway car.

Aboard adv. : Alongside; as, close aboard.

Aboard prep. : On board of; as, to go aboard a ship.

Aboard prep. : Across; athwart.

Example:
How many astronauts Aboard a space shuttle can actually fly it?

Taxonomy
Merriam-Webster Online Dictionary, Dictionary, webster, Word Games, Webster Online Dictionary with audio pronunciations, thesaurus, adverb, adjective, glossary, english, language, ESL, 14th century, vehicle, car, baseball, Unabridged Dictionary, Search Boxes.

Defination from the web:
aboard from the Merriam-Webster Online Dictionary with audio pronunciations, thesaurus, Word of the Day, and word games. ... a vehicle (as a car or ship) b : in or into a group, association, or organization <her second promotion since coming aboard> 3 baseball : on base ...