A consistent heuristic is a heuristic function whose estimated cost obeys a monotonicity condition across neighboring states. In pathfinding, consistency helps algorithms such as A* maintain optimality and avoid unnecessary re-expansion of nodes.
In the study of path-finding problems in artificial intelligence, a heuristic function is said to be consistent, or monotone, if its estimate is always less than or equal to the estimated distance from any neighboring vertex to the goal, plus the cost of reaching that neighbor.