In temporal compression of digital video, a keyframe is a frame that is stored in the video stream in full - that is, every pixel of the image area is stored. (The remainder of the stream is filled with delta frames.)
There are a few strategies in the codecs for picking the keyframes: A codec can use a constant keyframe interval (30 frames is pretty good for Indeo) or do the keyframes based on scene changes (this is what DivX 4 seems to do by default).
Keyframes are often important because they help people to seek to correct spot in the video stream. It's easier to seek around using keyframes because decoding of delta frames needs some interpolation.
In MPEG video, key frames are called I-frames (intraframes).
See also temporal compression.