on board vs aboard

on board and aboard both are adverbs.

Word	Noun	Adjective	Verb	Adverb
on board	No	No	No	Yes
aboard	No	No	No	Yes

As adverbs, on board and aboard are synonyms defined as:

on board (adverb)	aboard (adverb)
on a ship, train, plane or other vehicle	on first or second or third base
	on a ship, train, plane or other vehicle
	part of a group
	side by side