Definition from Wiktionary, the free dictionary
See also onboard
- (idiomatic) On or in a means of transportation.
- Even when I am on board the plane, I can never feel secure that my luggage is, too.
- (idiomatic) Joining in or participating.
- Is that new teammate properly on board yet?
- (idiomatic) Agreeing or supporting.
- It's a good idea, but let's see if we can get a few more of the management team on board.
Derived terms 
on or in a means of transportation
- On a vehicle or vessel; aboard
aboard — see aboard