A key frame (or keyframe) in animation and filmmaking is a drawing which defines the starting and ending points of any smooth transition. They are called "frames" because their position in time is measured in frames on a strip of film. A sequence of keyframes defines which movement the spectator will see, whereas the position of the keyframes on the film, video or animation defines the timing of the movement. Because only two or three keyframes over the span of a second does not create the illusion of movement, the remaining frames are filled with inbetweens.

In flash, a keyframe is similar in purpose. Keyframes are the only frames where symbols and shapes can be manipulated. While it is possible to produce a frame-by-frame animation out of nothing but a series of keyframes, many timeline animations use motion tweens and shape tweens which occur between keyframes. A keyframe is created by right-clicking a frame on the timeline and selecting "Insert Keyframe" or "Convert to Keyframe". Every layer has it's own series of keyframes. Keyframes can also hold ActionScript that will execute when the playhead crosses the frame.