Any procedure used in cryptography to convert plaintext into ciphertext (encrypted message) in order to prevent any but the intended recipient from reading that data.
Schematically, there are two classes of encryption primitives: public-key cryptography and private-key cryptography; they are generally used complementarily. Public-key encryption algorithms include RSA; private-key algorithms include the obsolescent Data Encryption Standard, the Advanced Encryption Standard, as well as RC4.
The Unix command crypt performs a weak form of encryption. Stronger encryption programs include Pretty Good Privacy and the GNU Privacy Guard.
Other closely related aspects of cryptograph include message digests.