An algorithm is a finite set of instructions which, if followed, accomplish a particular task. In addition every algorithm must satisfy the following criteria:

1. input: There are zero or more quantities which are externally supplied;
2. output: at least one quantity is produced;

3. definiteness: each instruction must be unambiguous;

4. finiteness: if we trace out the instruction of an algorithm, then for all cases the algorithm will terminate after a finite number of steps;

5. effectiveness: every instruction must be sufficiently basic that it can in principle be carried out by a person using only pencil and paper. It I snot enough that each operation be definite as in (iii), but it must also be feasible.