What is a condominium? It May Not be What You Think.

As a Realtor and appraiser, I am often asked what defines a condominium.  Some of the defining characteristics I have heard are:
1. A condominium does not have ownership of land associated with the unit.
2. A condominium shares one or more walls, or party walls, with another unit.
3. A condominium has a unit above and/or below it.

Though some or all of these factors can be true, there is only one reliable definition of a condominium, and that is form of ownership.   Condominiums are an individual interest in an apartment and an undivided interest in common areas contained in a condominium project.  It is ownership of the airspace whithin the unit only.  So, this is why a building that by all appearances looks like a duplex, can be considered a condominium.  Condominiums of course have other things that make them unique, such as a condominium declaration, which should be researched and understood if one is to best serve their clients whom want to buy or sell a condominium. 
Special tip regarding condominiums:  Other owners in a condominium project may have right of first refusal, in the case of an owner selling their individual condominium.  Something worth looking into prior to listing!