What is Cohabitation Agreement?

Legal Definition
A cohabitation agreement is a form of legal agreement reached between a couple who have chosen to live together (whether they are heterosexual or homosexual). In some ways, such a couple may be treated like a married couple, such as when applying for a mortgage or working out child support. However, in some other areas, such as property rights, pensions and inheritance, they are treated differently.

A cohabitation agreement contains documentation for a couple who want to live together in order to protect themselves from unnecessary cost and litigation should their cohabitation break down. They can clearly regulate their property rights and what arrangements might be made for mutual financial support, dealing with debt, caring for children, etc.

The agreement also, much like a prenuptial agreement, allows the individuals concerned to determine in advance who will keep specific assets and what will happen to assets that have been purchased jointly if they separate. This agreement is intended to bind both parties.
-- Wikipedia