A point in the sky located directly above a location and to which a vertical line would lead from that location. When a lot is located in a horizontal cadastral plan, it generally includes, unless otherwise indicated, the entire volume of air above it, up to the zenith.
By purchasing a condo (apartment) in a residential tower, you automatically become an owner in a vertical co-ownership. You can also be in a divided co-ownership, if you purchase a house (semi-detached or townhouse), built on the same lot than other individual homes. It is then called a horizontal co-ownership.