Definitions for "Keyable"
an element is said to be keyable if there is a method of separating it from its background video. One method for this separation is to create the element over negative black, which allows it to be keyed by a luminance or self key. Another (preferred) way is an external key which uses a device (CG, etc.) or matte reel to supply a separate key signal which cuts out the element. Alpha channels along with masks are created using computers to allow video to be keyed.
An element is said to be keyable if there is a method for separating it from the background video. One method is to put the element over negative black, which allows the element to be keyed by luminance. Another preferred way is to use a matte reel which supplies a key signal to cut out the element. This method is cleaner and more commonly used in edit suites which can handle a separate key signal.