The making public of what was private. Publication is a threshold — after it, the work belongs to its readers too.
Latin publicatio (a making public), from publicare (to make public property), from publicus (belonging to the people); publication is the specific act of crossing from one's own into everyone's.
The making public of what was private — a threshold, not a description. After it, the root's own claim applies: the work belongs to the people who read it as much as to the one who made it.