Aboard means on or into a ship, airplane, train, or other vehicle. When passengers walk onto a plane, they're going aboard. When a sailor climbs onto a ship, they're climbing aboard. The conductor might call out “All aboard!” to let passengers know the train is about to leave.

The word comes from nautical language, where aboard originally meant on a boat or ship. Today we use it for any vehicle that carries people: spacecraft, buses, even submarines. Astronauts are aboard the International Space Station. Students ride aboard the school bus.

You can also use aboard to mean joining a team or project. When a company hires someone new, they might say “Welcome aboard!” to show the person is now part of the group. It's like the difference between standing on the dock watching a ship and actually climbing on deck to join the crew.

Notice that aboard suggests being inside or on top of something that's moving or will move, not just near it. You stand beside a train, but you ride aboard it.