When two or more persons are equally owners of some property.
The unique aspect of joint tenancy is that as the joint tenancy owners die, their shares accrue to the surviving owner(s) so that, eventually, the entire share is held by one person.
A valid joint tenancy is said to require the "four unities": unity of interest (each joint tenant must have an equal interest including equality of duration and extent), unity of title (the interests must arise from the same document), unity of possession (each joint tenant must have an equal right to occupy the entire property) and unity of time: the interests of the joint tenants must arise at the same time.
See the article on Joint and Common Tenancies.