all aboard

WordReference Random House Learner's Dictionary of American English © 2025

a•board /əˈbɔrd/   adv., prep. 

1. on, in, or into (a ship, train, airplane, etc.):
   The ship sank, drowning all who were aboard. They went aboard the ship.

WordReference Random House Unabridged Dictionary of American English © 2025

a•board  (ə bôrd′, ə bōrd′), adv. 

1. on board;
   on, in, or into a ship, train, airplane, bus, etc.:
   to step aboard.
2. alongside;
   to the side.
3. Sport [Baseball.] on base:
   a homer with two aboard.
4. all aboard! (as a warning to passengers entering or planning to enter a train, bus, boat, etc., just before starting) Everyone get on!
5. into a group as a new member:
   The office manager welcomed him aboard.

prep. 

6. on board of;
   on, in, or into:
   to come aboard a ship.

• Middle English abord(e) (see a-¹, board), perh. conflated with Middle French a bord 1350–1400

Collins Concise English Dictionary © HarperCollins Publishers::

aboard / əˈbɔːd/ adverb , adjective , preposition (postpositive)

1. on, in, onto, or into (a ship, train, aircraft, etc)
2. alongside (a vessel)
3. all aboard! ⇒ a warning to passengers to board a vehicle, ship, etc