 In the context of search algorithms monotonicity (also called consistency) is a condition applied to heuristic functions.A heuristic "h(n)" is monotonic if, for every node "n" and every successor "n"' of "n" generated by any action "a", the estimated cost of reaching the goal from "n" is no greater than the step cost of getting to " n' " plus the estimated cost of reaching the goal from " n' ",

This is a form of triangle inequality, with "n", "n"', and the goal "Gn" closest to "n".Because every monotonic heuristic is also admissible, monotonicity is a stricter requirement than admissibility.