cryptography:
The art or science encompassing the principles and methods of transforming an intelligible message into one that is unintelligible, and then retransforming that message back to its original form.

plaintext:
The original intelligible message

ciphertext:
The transformed message

cipher:
An algorithm for transforming an intelligible message into one that is unintelligible by transposition and/or substitution methods

key:
Some critical information used by the cipher, known only to the sender and receiver

encipher (encode):
The process of converting plaintext to ciphertext using a cipher and a key

decipher (decode):
The process of converting ciphertext back into plaintext using a cipher and a key

cryptanalysis:
The study of principles and methods of transforming an unintelligible message back into an intelligible message without knowledge of the key. Also called codebreaking.

cryptology:
Both cryptography and cryptanalysis

code:
An algorithm for transforming an intelligible message into an unintelligible one using a code-book.