A cryptosystem or cipher system is a method of disguising messages so that only certain people can see through the disguise. Cryptography is the art of creating and using cryptosystems. Cryptology is the study of both cryptography and cryptanalysis. The original message is called a plaintext. The disguised message is called a ciphertext.
Encryption means any procedure to convert plaintext into ciphertext.
Decryption means any procedure to convert ciphertext into plaintext. A cryptosystem is usually a whole collection of algorithms. It's natural to say that n is the key here. The people who are supposed to be able to see through the disguise are called recipients. Other people are enemies, opponents, interlopers, eavesdroppers, or third parties.