A fixture is something that is stuck or "fixed" somewhere, like the plumbing fixtures in your bathroom or the guy who is a fixture in this favorite diner, always sitting in the third booth from the door.
When a house is for sale, it's generally assumed that the fixtures — the kitchen sink, the fireplace, or a ceiling fan — are included with the house. Fixtures tend to be the objects that are fixed, or securely fastened, in place. A regular patron or customer can also be called a fixture, like the fixtures at the local diner who never fail to show up for the early bird special. Fixture, modeled after mixture, comes from the Latin root fixus, "immovable, established, or settled."