Possessio, in its primary sense, is the condition or power by virtue of which a man has such a mastery over a corporeal thing as to deal with it at his pleasure, and to exclude other persons from meddling with it. This condition or power is detention; and it lies at the bottom of all legal senses of the word 'possession.' This possession is no legal state or condition, but it may be the source of rights, and it then becomes possessio in a juristical or legal sense. Still, even in this sense it is not in any way to be confounded with property (proprietas). A man may have the juristical possession of a thing without being the proprietor, and a man may be the proprietor of a thing without having the juristical possession of it, and consequently without having the detention of it (Dig. 41, tit. 2, s. 12). Ownership is the legal capacity to operate on a thing according to a man's pleasure, and to exclude everybody else from doing so. Possession, in the sense of detention, is the actual exercise of such a power as the owner has a right to exercise. The term possessio occurs in the Roman jurists in various senses. There is possessio generally, possessio civilis, and possessio naturalis.
Possessio denoted, originally bare detention; but this detention, under certain conditions, becomes a legal state inasmuch as it leads to ownership through usucapio. Accordingly the word possessio, which required no qualification so long as there was no other notion attached to possessio, requires such qualification when detention becomes a legal state. This detention, then, when it has the conditions necessary to usucapio, is called possessio civilis, and all other possessio as opposed to civilis is naturalis, Sand. Just.