On board; into or within a ship or boat; hence, into or within a railway car.

"We all climbed aboard."

On or onto a horse, a camel, etc.

"To sling a saddle aboard."

On base.

"He doubled with two men aboard, scoring them both."

Into a team, group, or company.

"The office manager welcomed him aboard."

Alongside.

"The captain laid his ship aboard the enemy's ship."

On board of; onto or into a ship, boat, train, plane.

"We all went aboard the ship."

Onto a horse.

Across; athwart; alongside.