An agreement or coincidence of certain qualities in the title of a joint estate or an estate in common.
In a joint estate there must exist four unities; that of interest, for a joint-tenant cannot be entitled to one period of duration or quantity of interest in lands, and the other to a different; one cannot be tenant for life, and the other for years: that of title, and therefore their estate must be created by one and, the same act; that of time, for their estates must be vested at one and the same period, as well as by one and the same title; and lastly, the unity of possession: hence joint-tenants are seised per my et per tout, or by the half or moiety and by all: that is, each of them has an entire possession, as well of every parcel as of the whole.
Coparceners must have the unities of interest, title, and possession.
In tenancies-in-common, the unity of possession is alone required.
UNITY OF POSSESSION
This term is used to designate the possession by one person of several estates or rights. For example, a right to an estate to which an easement is attached, or the dominant estate, and to an estate which an easement encumbers, or the servient estate, in such case the easement is extinguished. But a distinction has been made between a thing that has being by prescription, and one that has its being ex jure naturae; in the former case unity of possession will extinguish the easement; in the latter, for example, the case of a water course, the unity will not extinguish it.