adverb
On board; into or within a ship or boat; hence, into or within a railway car.
On or onto a horse, a camel, etc.
(baseball) On base.
Into a team, group, or company.
(nautical) Alongside.
preposition
On board of; onto or into a ship, boat, train, plane.
Onto a horse.
(obsolete) Across; athwart; alongside.