1.PrepositionOn or in (a means of transportation).
"On board his boat were various firearms but not much food.; Even when I am on board the plane, I can never feel secure that my luggage is, too."
2.AdjectiveJoining in or participating.
"Is that new teammate properly on board yet?"
3.AdjectiveAgreeing or supporting.
"It's a good idea, but let's see if we can get a few more of the management team on board.; Without management on board, the project has little chance of success."
4.AdjectiveInto itself or oneself.
5.InterjectionThe stereotypical cry of pirates when boarding a ship for close quarters combat.