Cement is a material that's used to build very strong, hard surfaces and structures. Cement is an ingredient in both mortar, which holds bricks together, and concrete, with which dams and roads and buildings are constructed.
Cement begins as a powder made of clay and limestone, and after it's mixed with water and, often, sand, it's used to strengthen and bind things — as the mixture dries, it becomes hard and solid. A version of cement has been around since the third century BCE, in Mesopotamia. The word comes from the Latin root cæmenta, "stone chips for making mortar," from caedere, "to cut or chop."