Original: In soil science, humus refers to any organic matter that has reached a point of stability, where it will break down no further and might, if conditions do not change, remain essentially as it is for centuries, if not millenniaWhitehead, D.C., Tinsley, J., 2006.
Simplified: In the earth sciences, "humus" means any organic matter which has reached a point of stability, where it will break down no further and might, if conditions do not change, remain essentially as it is for centuries, or millennia.