"For the Romans, a villa was always a residence located outside the city walls. Beyond that, the concept is so all-encompassing and vague that as early as 37 B.C. Varro could contrast a simple farming villa with a villa in the Campus Martius full of works of art. This contrast can only be understood in terms of an historical process. The villa was for a very long time the place where a farmer lived and worked. A farm consisted of the villa and the ager, the land and property. Both together can also be called a praedium or fundus. Both concepts were used down to late antiquity. For a more precise designation, one always connected the praedium with a place name, for example praedium Tusculanum. Cicero thus speaks in abbreviated form of his Tusculanum, Pompeianum, Puteolanum, in order to designate his various villas. On the other hand, fundus is used in connection with the first owner, whose name was preserved for a very long time. With the expansion of urban luxury to the living quarters of the villa a distinction arose between the villa urbana and the working part, the villa rustica. In extreme cases the villa rustica could even be totally lacking. This happened in the vicinity of Rome and on the coast. The old designation villa for farmhouse is maintained until late antiquity, so that a wide range of possible residences outside the city can be called a villa. Another complicating factor is the similarity of the villa to the horti, the gardens located before the city wall. From the first century B.C., these gardens (which from early times were used, along with the town house, or domus, as a temporary residence) were furnished in a similar way to the more distant country villas. A sharp distinction can no longer be drawn, for example when Pompey's residence in his garden is called a villa (Asconius, in Pro Milone XXI). Theoretically, at least, in these horti the gardens were more noticeable than the buildings. This is shown by the pertinent criticism of Pliny (NH XIX.50).
"Finally, in the middle of the first century A.D., the expression praetorium appears next to villa as the designation for the residence. It refers originally to the headquarters of the commander of a Roman legion and was perhaps at first used for imperial villas. It quickly took on a more general meaning.
"By villa are thus meant different kinds of buildings with very different functions. The Roman villa received its constant characteristic (at least in central Italy) through the way it was used, which distinguishes it from later related phenomena. The prerequisite for its origin is the concentration of property into the hands of a small class, which often owned enormous estates. Usually, these were not located in a single place but were widely scattered. The members of this upper class composed the most important government bodies in Rome, but often came from areas far from the capital city. Unlike the medieval and late-antique feudal lords, who were mainly resident on their estates, the Roman upper class tended to live in Rome itself. They therefore usually visited their villas for only a part of the year, and for the choice of season, the climate was generally decisive. The Roman villa is historically significant because of the fact that its owners visited them not simply for country recreations like hunting (as is known from other periods). Rather, the Roman villas were sites of intensive cultural life, places where art and literature could be enjoyed in a way that was more free and more relaxed than in Rome. One can even speak of the villa as an 'artistic and educational landscape'...."
--Harold Mielsch, Die römische Villa. Architetktur und Lebensform (Verlag C. H. Beck, Munich, 1997) 7-8 (translated by Bernard Frischer)