Backtracking is when a search or related algorithm returns to previous state. This maybe because the algorithm has got stuck, for example when a tree search encounters a node that is infeasible, that is does not satisfy all of the constraints for an acceptable solution. Alternatively, when the algorithm has found some form of a soluution, it may want to look for others.
Used on pages 23, 33, 67, 72, 75, 79, 348