Definitions

Plaintext: the original message (before encryption)
Ciphertext: the coded message (after encryption)
Enciphering/encryption: the process of converting from plaintext to ciphertext
Deciphering/decryption: restoring the plaintext from the ciphertext
Secret key: The secret key is also input to the encryption algorithm. The key is a value independent of the plaintext and of the algorithm. The algorithm will pro- duce a different output depending on the specific key being used at the time.
Cryptography: The area of study of the many schemes used for encryption
Cryptographic system/cipher: A scheme
Cryptanalysis: Techniques used for deciphering a message without any knowledge of the enciphering details
Cryptology: The areas of cryptography and cryptanalysis

Symmetric Cipher Model

There are two requirements for secure use of conventional encryption:

A strong encryption algorithm
Sender and receiver must have obtained copies of the secret key in a secure fashion and must keep the key secure