A scheme for SOLving a series of sub-problems each of which may have multiple possible SOLutions and where the SOLution chosen for one sub-problem may affect the possible SOLutions of later sub-problems. To SOLve the overall problem, we find a SOLution to the first sub-problem and then attempt to recursively SOLve the other sub-problems based on this first SOLution. If we cannot, or we want all possible SOLutions, we backtrack and try the next possible SOLution to the first sub-problem and so on. Backtracking terminates when there are no more SOLutions to the first sub-problem. This is the algorithm used by logic programming languages such as Prolog to find all possible ways of proving a goal. An optimisation known as "intelligent backtracking" keeps track of the dependencies between sub-problems and only re-SOLves those which depend on an earlier SOLution which has changed. Backtracking is one algorithm which can be used to implement nondeterminism. It is effectively a depth-first search of a problem space. (1995-04-13)