The turning of a thing into a form only the intended holder can read. Encryption protects contents and never the fact of sending.
Twentieth-century formation from Greek kryptos (hidden), en- (a causative prefix) plus -tion, built as the counterpart to decrypt.
Its root is the same hidden that gives crypt and cryptic, and the word itself was coined only to give decrypt something to undo. What is encrypted is made unreadable on purpose, by a hand that also holds the one way back.