In coding theory, a cyclic code is a block code, where the circular shifts of each codeword gives another word that belongs to the code. They are error-correcting codes that have algebraic properties that are convenient for efficient error detection and correction. Let be a linear code over a finite field (also called Galois field) of block length . is called a cyclic code if, for every codeword from , the word in obtained by a cyclic right shift of components is again a codeword. Because one cyclic right shift is equal to cyclic left shifts, a cyclic code may also be defined via cyclic left shifts. Therefore, the linear code is cyclic precisely when it is invariant under all cyclic shifts. Cyclic codes have some additional structural constraint on the codes. They are based on Galois fields and because of their structural properties they are very useful for error controls. Their structure is strongly related to Galois fields because of which the encoding and decoding algorithms for cyclic codes are computationally efficient. Cyclic codes can be linked to ideals in certain rings. Let be a polynomial ring over the finite field . Identify the elements of the cyclic code with polynomials in such that maps to the polynomial thus multiplication by corresponds to a cyclic shift. Then is an ideal in , and hence principal, since is a principal ideal ring. The ideal is generated by the unique monic element in of minimum degree, the generator polynomial . This must be a divisor of . It follows that every cyclic code is a polynomial code. If the generator polynomial has degree then the rank of the code is . The idempotent of is a codeword such that (that is, is an idempotent element of ) and is an identity for the code, that is for every codeword . If and are coprime such a word always exists and is unique; it is a generator of the code. An irreducible code is a cyclic code in which the code, as an ideal is irreducible, i.e. is minimal in , so that its check polynomial is an irreducible polynomial.