What part of speech is aboard?

"Aboard" can be a few different parts of speech depending on how it's used:

* Preposition: When it indicates location, like "The passengers were aboard the ship."

* Adverb: When it means "on board a vehicle," like "They went aboard the train."

* Adjective: In rare cases, it can act as an adjective, though it's less common. For example, "The aboard passengers cheered."

Let me know the full sentence where you encountered "aboard," and I can tell you for sure what part of speech it is!