A fixture, as a legal concept, means any physical property that is permanently attached (fixed) to real property (usually land). Property not affixed to real property is considered chattel property. Fixtures are treated as a part of real property, particularly in the case of a security interest. A classic example of a fixture is a building, which, in the absence of language to the contrary in a contract of sale, is considered part of the land itself and not a separate piece of property. Generally speaking, the test for deciding whether an article is a fixture or a chattel turns on the purpose of attachment. If the purpose was to enhance the land, the article is likely a fixture; if the article was affixed to enhance the use of the chattel itself, the article is likely a chattel.

Chattel property is converted into a fixture by the process of attachment. For example, if a piece of lumber sits in a lumber yard, it is a chattel. If the same lumber is used to build a fence on the land, it becomes a fixture to that real property. In many cases, the determination of whether property is a fixture or a chattel turns on the degree to which the property is attached to the land. For example, this problem arises in the case of a trailer home. In this case, the characterization of the home as chattel or realty will depend on how permanently it is attached, such as whether the trailer has a foundation.

The characterization of property as a fixture or as chattel is important. In most jurisdictions, the law respecting the registration of security against debt, or proof that money has been lent on the collateral of property, is different for chattels than it is for real property. For example, in the province of Ontario, Canada, mortgages against real property must be registered in the county or region's land titles office. However, mortgages against chattels must be registered in the province-wide registry set up under the Personal Property Security Act.

In the case of a trailer home, whether it is a fixture or chattel has a bearing on whether a real property mortgage applies to the trailer. For example, most mortgages contain a clause that forbids the borrower from removing or demolishing fixtures on the property, which would lower the value of the security. However, there have been cases where lenders lend money based on the value of the trailer home on the property, where that trailer is later removed from the property. Similarly, a chattel mortgage granted to allow a person to purchase a trailer home could be lost if the trailer is later attached to real property.

The law regarding fixtures can also cause many problems with property held under a lease. Fixtures put in place by the tenant belong to the landlord if the tenant is evicted from the property. This is the case even if the fixture could have legally been removed by the tenant while the lease was in good standing. For example, a chandelier hung by the tenant may become the property of the landlord. Although this example is trivial, there have been cases where heavy equipment incorporated into a plant has been deemed to have become fixtures even though it was sold as chattels.

Because the value of fixtures often exceeds the value of the land they are affixed to, lawsuits to determine whether a particular item is a chattel or a fixture are common. In one case in Canada, a provincial government argued that a huge earth dam was a chattel, as it was only held in place by gravity and not by any type of affixation (the claim was rejected). In a sale of land, fixtures are treated as part of the land, and may not be removed or altered by the seller prior to the transfer of the land.

Fixtures are known in civil law as essential parts.