Decryption is the inverse of encryption: in decryption, encrypted ciphertext becomes plaintext.
Decryption is the reverse mathematical transformation of encryption. The precondition is that the encryption function is reversible.
Decryption must be possible only for the legal recipient of the transmitted message, and not for other persons. In some cases, decryption itself is withheld from the sender of the message. Decryption of the ciphertext into plaintext can only be done with the correct key.