2. n. 
The process by which material weathered from rocks is transported by wind, water, ice, or abrasive solid particles, or by mass-wasting, as in rock falls and landslides.
3. n. 
The wearing away of material, usually rock or steel, by the continuous abrasive action of a solids-laden slurry. For erosion to occur usually requires a high fluid velocity, on the order of hundreds of feet per second, and some solids content, especially sand. Erosion may also occur in gas streams, again assuming the presence of sand particles. It is usually difficult to erode the wellbore wall significantly with drilling mud alone due to its relatively low velocity and high viscosity. There is also a dramatic "self-limiting" effect because even slight enlargement of the original gauge wellbore dramatically decreases fluid velocities.