Aboard means on or onto a ship, a plane or a train. Come aboard, welcome aboard, all aboard.
All aboard is the call before departure and welcome aboard has become a greeting to anybody joining a team, which is the metaphor everybody uses without noticing. The board in it means the side of a ship. Note that a- here means on, as in ashore and afloat.
1adverbaboardOn or onto a ship, a plane or a train.
“All the passengers were aboard, and the ferry left the harbour.”
aboard is a position on a vessel.
A- plus board, where board means the side of a ship. Ashore and afloat are built the same way.
Middle English
abord
English, today
aboard
Built from
From Middle English abord, from a- (“on”) + bord (“board, side of a ship”); equivalent to a- + board.
Etymology from Wiktionary, CC BY-SA