What is Unity?

Legal Definition
Estates. An agreement or coincidence of certain qualities in the title of a joint estate or an estate in common.

2. 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. 2 Bl. Com. 179-182; Co. Litt. 188.

3. Coparceners must have the unities of interest, title, and possession.

4. In tenancies in common, the unity of possession is alone required. 2 Bl. Com. 192; 2 Bouv. Inst. n. 1861-83. Vide Estate in Common; Estate in Joint-tenancy; Joint-tenants; Tenant in Common; Tenants, Joint.
-- Bouviers Law Dictionary
Legal Definition
In the law of estates. The peculiar characteristic of an estate held by several in joint tenancy, and which is fourfold, vis., unity of interest, unity of title, unity of time, and unity of possession. In other words, joint tenants have one and the same interest, accruing by one and the same conveyance, commencing at one and the same time, and held by one and the same undivided'possession. 2 Bl. Comm. 180. '
See also
-- Black's Law Dictionary