On board; into or within a ship or boat; hence, into or within a railway car.
"The passengers stepped aboard the ferry to begin their journey across the lake."
In plain English: Aboard means being on, inside, or part of a ship, plane, train, or other vehicle.
"We will be aboard the train by noon if you hurry."
Usage: Use aboard as an adverb to indicate movement onto or presence inside any vehicle such as a ship, plane, train, or bus. It is interchangeable with on board when describing location but functions grammatically without needing the preposition in before it.
On board of; onto or into a ship, boat, train, plane.
"The passengers walked aboard the cruise ship just as it began to sail away from the dock."
Aboard comes from Middle English, combining the prefix a- meaning "on" with bord, which referred to the side or deck of a ship. Essentially, the word originally meant being on the board or side of a vessel.