共有，相对於单独所有而言，意指数人共同享有一物之所有权，此数人称为共有人，该物称为共有物。若共有人所共有者为权利等无体财产权（例如债权）时，则为准共有。共有按形态又可分为分别共有和公同共有两种，日耳曼法上尚有村落共同体之共有型态称为总有，不过现已不存于法律规定之中。原则上，单独所有物之所有权为较单纯且不易生纷争之所有型态，因此共有关系并不被乐见其长久存续，而各国法律之规定也多倾向尽可能消灭共有关系，使物回复单独所有之型态。

A concurrent estate or co-tenancy is a concept in property law which describes the various ways in which property is owned by more than one person at a time. If more than one person own the same property, they are referred to as co-owners, co-tenants or joint tenants. Most common law jurisdictions recognize tenancies in common and joint tenancies, and some also recognize tenancies by the entirety. Many jurisdictions refer to a joint tenancy as a joint tenancy with right of survivorship, and a few U.S. states treat the phrase joint tenancy as synonymous with a tenancy in common.