The phrase “all aboard!” is a commonly used expression that is often used as a command to invite or encourage someone to join others in a particular activity or endeavor. The missing word in this phrase is “all“, which completes the expression “all aboard“. This phrase is particularly common in the context of transportation, where it might be used to prompt passengers to get on a train, bus, or other mode of travel. Additionally, the phrase may also be used in other contexts where people are gathering in a group or preparing to embark on a shared experience. Regardless of the context, the phrase “all aboard” is a clear and unambiguous invitation for people to come together and take part in something as a group.