A ceiling is a part of a house that separates the roof from the room, or one floor from another. Visible ceilings were introduced in The Sims 2: Apartment Life expansion pack. Before that, ceilings were assumed to be present, but were not visible.
In Apartment Life, a ceiling can be created above any room by placing floor tiles above it, and the player can change the ceiling to be whatever pattern of flooring they wish. Ceilings are also automatically created in The Sims 3, as this feature carried over. Ambitions allows ceilings to be painted.
Ceilings are also visible in The Sims 2: Pets (Console).