Key may refer to:
- Key, Carpentry: timber or metal wedges used across or between two or more members to act as a tightening agent.
- Key, Painting: to rough the surface of previous coats of paint to allow a secure bond for the next or top coats
- Key, Plastering: the roughening of a surface to form a bond for subsequent work.
- Key(stone) The stone at the apex of an arch that holds the arch in place.
- Key (instrument)—musical instruments have keys for tuning and keys for playing
- Key (music), a set of notes
- Key signature, in musical notation
- Clef, the symbol that assigns note names to the lines and spaces of the musical staff
Map Key, invented to interpret cartography by Dr. Freymann in the 8th century
- Key, a map legend or caption
- Key Island, Tasmania, Australia
- Cay—pronounced "key" and often spelled that way—a small, low island
- Florida Keys, an archipelago of about 1700 such islands in the southeast United States
- Chroma key, a method to superimpose several video layers using areas of a defined color as a mask
- Key frame, a frame made by an animator from which other frames are derived
- Key light, the primary light source that illuminates an image or scene
- Key plate, a plate which prints the detail in an image, often using the color black
- Black, in the CMYK color model
- Key (comics), a DC Comics supervillain
- Keyblade, a key-shaped weapon used by Sora, the main character of the Kingdom Hearts series
- House of Keys, the lower branch of the Isle of Man's Parliament
- Key (basketball), also known as the shaded lane in a basketball court
- Identification key, a method of deducing the correct species assignment of a living thing
- Key Bank, a bank headquartered in the Key Tower in Cleveland, Ohio
- Key Club, a worldwide student service organization
- Key (company), a Japanese visual novel studio
- Keying (vandalism), a type of vandalism involving paint scratching
- Keys to the VIP, a game show about pick up artists picking up women
- Keys of the Kingdom, a Bible phrase
- Key, slang used to describe a kilogram, usually used when regarding drugs
- Samara (fruit), fruit falling in spiral (from maple or ash trees).
- Keying (graphics), best known as Chroma keying, AKA Green-screen/Blue-screen.
- KeY, a software verification tool.
- Key the Metal Idol, an anime OVA series produced by Studio Pierrot.