adjective
Carried or used on or in a vehicle or vessel.
(by extension, figurative) Being a part of, being included in, participating in.
adverb
On or in a vehicle or vessel; aboard; on board.
verb
(figurative) To become a part of a group; to incorporate (someone) into a group.
(figurative) To begin to use a product or service; to take (someone) on as a new customer of a product or service.