"The measure of steepness or the degree of inclination of a feature relative to the horizontal plane." (Lemoine, Douglas and Cotton 2012)
Lemoine, Anne, John Douglas, and Fabrice Cotton. "Testing the applicability of correlations between topographic slope and VS 30 for Europe." Bulletin of the Seismological Society of America 102, no. 6 (2012): 2585-2599.