A joist is a support that strengthens floors and ceilings in a house. Joists run horizontally, connecting with the studs in walls to give buildings a solid frame.
You can't see the joists in a finished house or building, because they're inside the foundation, walls, or ceiling. Joists are usually made of wood, although they can also be concrete or steel, materials strong enough to provide support. Ceiling joists often connect to beams, which tend to be larger and run perpendicular to joists. Joist comes from the Old French word giste, "beam supporting a bridge," from the Latin iacere, "to rest."