A legal fiction left over from common law--a creation of a unity for joint tenancy or ownership. The four unities are interest, possession, time, and title. See alsojoint tenancy. unity of interest. Requires that each and every joint tenant's interests are identical in kind, breadth, and time. unity of possession. The necessity that each joint tenant must be entitled to possess the whole property in its entirety. unity of time. The necessity that all joint tenants' individual interests must v...