1. On a ship, train, plane or other vehicle
   "The passengers were already aboard when the captain made the announcement";
   - on board
    
2. (baseball) on first, second or third base
   "Their second homer with Bob Allison aboard"
    
3. Side by side
   "anchored close aboard another ship";
   - alongside
    
4. Part of a group
   "Bill's been aboard for three years now"

1. On board of
   "We all went aboard the ship"

Encyclopedia: Aboard