Why is Mars soil called "Regolith"? 

On Earth, we're usually walking on or surrounded by soil. The soil we're familiar with is a mixture of several different components. There's broken down rock, decayed organic matter, dried leaves, bacterial colonies, fungi, worms and insects - a robust mixture of biological, organic, and inorganic elements. On Earth, soil is its own ecosystem, essential to the function of the biosphere.

Biology is unique to Earth, and it makes up a big part of our soil. From nitrogen-fixing bacteria to cow manure, every level of the trophic chain leaves something behind in the substrate it lives on. In particular, the actions of fungi and bacteria change not only the chemical composition and reactivity of the soil, but an also have an effect on the physical characteristics of the inorganic element in the soil.

Except the gas giants, many of the larger objects in the Solar System are partially covered with loose, broken down material, in some cases eroded and in some cases accumulated by gravity. To prevent confusion, planetary scientists needed a term other than "Soil" to refer to loose material that covers the surface of many planetary bodies. The term Regolith refers to unconsolidated rocky material covering bedrock, regardless of planet.

Mars regolith, then, is the loose, eroded material that covers parts of the surface of Mars. On Earth's Moon, Lunar regolith covers much of the surface. Venusian regolith, Europan regolith, and Plutonian regolith all exist and are all distinct from each other. Go outside and pick up a handful of dirt from your garden - that's Earth regolith.