同居（英语：Cohabitation）一般指，两个具备亲密关系但是没有结婚的人居住在一起。

同居大部分指的是和异性伴侣的亲密关系，两个人之间有着爱情上的链接，大多数也有性关系，两人之间没有婚姻关系。可据此分为「有性同居」与「无性同居」。同居者可能是同性，但不一定等同于两个同性恋者居住在一起。

一些同居伴侣共同育有小孩，创建家庭，但并没有结婚，故可说是「无婚姻之名，有婚姻之实」。

有时候泛指不存在恋爱或性关系的多人合住一处，即室友（英语：Roommate），此种情况多称为“同住”或“同室”。

近年选择同居的伴侣增加，有些伴侣在同居一段时间后才会选择结婚，因此同居成为伴侣间彼此适应与磨合的方式；但也有些伴侣长期维持同居的状态而没有结婚。

A domestic partnership is an interpersonal relationship between two individuals who live together and share a common domestic life but are not married (to each other or to anyone else). The term is not used consistently, which results in some inter-jurisdictional confusion. Some jurisdictions, such as Australia, New Zealand, and the American states of Oregon, Washington (only available if one of the parties is 62 or older), Nevada, Wisconsin, Maine, and California, use the term "Domestic Partnership" to mean what other jurisdictions call Civil Union, Civil Partnership, or Registered Partnership. Other jurisdictions use the term as it was originally coined, to mean an interpersonal status created by local municipal and county governments, which provides an extremely limited range of rights and responsibilities.