An algorithm is a rule that, if applied appropriately, guarantees a solution to a problem. We can use an algorithm even if we cannot understand why it works.
For example, you may know that you can find the length of the third side of a right triangle by using the formula a2+ b2=c2, although you may not have the foggiest notion of the mathematical principles behind the formula.
Algorithm is a widely-used scientific procedure for solving a problem which (unlike a HEURISTIC) is guaranteed to find the solution, or the best solution possible, even if the user doesn't know how it works.