The collection of "loose" solid matter covering the
bedrock of any
solid planet,
moon, or other astronomical body (pehaps a large
asteroid) massive enough to trap it in its gravitational field.
Composition of regolith varies widely between planets, and between different regions on the same planet. The regolith most familiar to us is ordinary soil; also included in this classification are larger rocks, volcanic ash and dust, organic matter such as peat, fragments from meteorite bombardments, and many others. Regolith distribution is also far from uniform, ranging from more than 100 meters thick to bare rock in different locations on Earth.
On a planet with a well-formed atmosphere (such as Earth), regolith is primarily the result of weathering and erosion. On an unprotected body such as Mars or the Moon, however, regolith forms by the collective effects of meteorite impacts over time, since no atmospheric friction exists to shield the planet/moon from incoming objects or to wear down the surface with winds.