In cryptography, what is cipher?

A cipher is an algorithm for performing encryption or decryption—a series of well-defined steps that can be followed as a procedure. In cryptography, a cipher transforms plaintext (readable data) into ciphertext (encoded data) to secure the information, making it unreadable without the proper key or method to decode it. There are various types of ciphers, including symmetric (where the same key is used for both encryption and decryption) and asymmetric (which uses a pair of keys).