Encryption Key
A piece of information used by an encryption algorithm to scramble or unscramble data.
An encryption key↗ is the piece of information, a long string of essentially random data, that an encryption↗ algorithm uses to scramble data into an unreadable form, and to unscramble it again. Without the correct key, encrypted data looks like meaningless noise, even if someone has full access to the file itself.
Keys come in two broad flavors: symmetric, where the same key locks and unlocks the data, and asymmetric (public-key), where a public key encrypts and only a matching private key can decrypt. The strength of encryption depends heavily on keeping keys secret and long enough to resist brute-force guessing, losing control of a key is functionally the same as losing the protection encryption was supposed to provide.