On, onto, within
ETYM Pref. a- on, in + board.
1. On a ship, train, plane or other vehicle; SYN. onboard.
2. (Baseball) On first or second or third base; SYN. on base.
3. Part of a group.
4. Side by side; SYN. alongside.

1. Alongside
2. On, onto, or within a vehicle (as a car or ship)
3. In or into a group, association, or organization
4. Baseball; on base