An Admissible Heuristic is a heuristic function that never overestimates the true minimum cost of reaching a goal. In pathfinding and search algorithms such as A*, admissibility helps preserve optimality of the solution.
In computer science, specifically in algorithms related to pathfinding, a heuristic function is said to be admissible if it never overestimates the cost of reaching the goal, i.e. the cost it estimates to reach the goal is not higher than the lowest possible cost from the current point in the path.