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. As of Ambitions, ceilings can be painted. In The Sims 4, ceilings are automatically placed if a room is created, but players can remove and edit them the way they like. Ceilings can not be painted in The Sims 4.
Ceilings are also visible in The Sims 2: Pets (Console).