The word "rock" comes from the Old English word "rocca", which is thought to be related to the Celtic word "creg", meaning "rock". The word "rock" has been used in English since the 13th century to refer to a solid, inorganic substance that is found in the ground. Geologists use the term rock to describe naturally occurring, solid aggregates of one or more minerals. This definition distinguishes rocks from loose sediments found at the surface of the Earth and manufactured materials like concrete.