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Meaning of directional derivative

direc'tional deriv'ative

Pronunciation: [key]
— Math. Math.
  1. the limit, as a function of several variables moving along a given line from one specified point to another on the line, of the difference in the functional values at the two points divided by the distance between the points.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.