An omnibus is another word for a bus, as in a large vehicle carrying lots of passengers. Other names are autobus and coach.
This word has bus in it, and that's the main meaning of omnibus. As a book, an omnibus is collection of articles either all on the same subject or written by a single author. An omnibus of Joyce Carol Oates would fill more shelves than all of Shakespeare’s plays. A third meaning is something that covers a lot of different subjects at once, like an omnibus bill that has legislation about gun control, transportation, and parking meters.