A Berth
Yes, it can be. But it can also be an adverb ("Come aboard"). It refers to riding in or on a vehicle, such as a ship, plane, train, or bus.
A Jonah (from the biblical Jonah and the whale) was the name given to somebody aboard who was perceived as being a jinx. Seamen are a superstitious bunch even today, so don't ever whistle aboard a ship!
The Mongolia
Yes, it is one word "aboard" -- as an adverb or preposition, it means "on board" a vehicle such as a bus, train, plane, or ship.
The Beagle
Buster Keaton
H.M.S Beagle
One boards by transferring - from shore, ground or other surface - to a ship (or other craft).On setting both feet upon the craft, one has boarded and so is now on board or aboard; the terms mean the same.The ship or other craft is assumed, and it isn't essential to mention it again if it's already been made clear what one is boarding and from where.
buson
the same as on a ship, the galley
There are several dictionary definitions for aboard. They include being aboard a ship, boat, train or other vehicle, being alongside or at the side of something, being brought into a group, and getting on base in the sport of baseball.
It is about 103 people aboard the Mayflower ship!