aŭtobuso

This is the best morphological accident in the chapter. The horse-drawn public carriages of 1820s France took the name omnibus, Latin for "for everybody", because that was the point of them: anyone could get on. Omnibus is not a noun in Latin at all - it is the dative plural of omnis, all, so the -bus is a case ending, the way the -s in "cats" is. English then clipped the word from the front, kept the grammatical suffix, and made it the name of the vehicle. Every bus in the world is named after a Latin inflection somebody misunderstood.

Once bus was a word it became productive, and the motorised version was an auto-bus. Esperanto has both halves in its own right: aŭto and buso are separate pages in this course, and aŭtobuso reads as a plain compound of the two even though that is not how it was actually built.

Which to use? Everyday Esperanto says buso, the same way everyday English says bus rather than motor omnibus. Aŭtobuso is the full, slightly formal form - timetables, official notices, the sort of prose that also says "commence" instead of "start". Both are correct; neither will be misunderstood. The related family is aŭtobusa (a bus route is an aŭtobusa linio), aŭtobushaltejo for a bus stop, and busisto for the driver, though ŝoforo does that job just as well.