Terminology
A sender wants to send a message to a receiver securely -- wants to make sure no eavesdropper can read the message.
Plaintext ----> Encryption -----------> Decryption ----> Plaintext
M E(M) C=E(M) D(C) M=D(C)
plaintext -- original message
encryption -- process of disguising message so as to hide its substance
ciphertext -- encrypted message
decryption -- process of turning ciphertext back into plaintext.
cryptography -- art and science of keeping messages secure
cryptanalysis -- art and science of breaking ciphertext