A film production term, popularized in Hollywood, that denotes a low-contrast
lighting style. The "high-key" is meant to refer to the supposed emotional tone that is produced by this style of lighting. A high-key film or scene tends to be "bright and cheery," or at least "safe," whereas a low-key
scene tends to be unsettling or threatening.
The low contrast in this style of lighting is produced by the fact that there is little difference in brightness between the various lights used in production. The result is that in the finished film there are no radical distinctions between light and shade... rather there is an even tone.
But consider the role of the key light in lighting a scene, and you see how confusing this term can be. The key light is the brightest light on the set, the one which serves as the principal source of light, whether it is justified by an element in the scene, such as a bright window or lamp, or it is purely an artificial device. In "high-key lighting," the key light is not significantly brighter than any of the other lights, and it is generally darker than the key light used in low-key lighting.