key

Key is a small machine element used to operate locks and security devices. Physical keys are typically metal blades with notches that align with a lock's pins or wafers, allowing turning of the cylinder. Keys may be single-use for one lock or issued as master keys that open several locks. Modern keys often include security features such as restricted duplication or integrated electronics in smart keys.

In cryptography, a key is information used by an algorithm to transform plaintext into ciphertext and back again. Keys can be symmetric, where the same secret key is used for encryption and decryption, or asymmetric, using a public key for encryption and a private key for decryption. Key management encompasses generation, distribution, storage, rotation, and revocation to maintain security.

In Western music, a key denotes the scale and tonal center around which a piece is organized. Keys are characterized as major or minor, and indicated by a key signature showing which notes are consistently sharp or flat. Pieces may modulate to different keys; related keys share the same key signature.

In maps and diagrams, a key (also called a legend) explains the symbols, colors, and line styles used to convey information. The key helps readers interpret distances, elevation, land use, or other data represented on the page.

The term key also appears in computing as a key on a keyboard or keypad; and metaphorically as something essential or central, as in "the key to success." The word has multiple senses across fields, all related to access, meaning, or center of importance.