![1girl animal_print animation_paper belly blanket breasts bunny bunny_print cleavage color_trace commentary crop_marks futon groin highres key_frame kill_la_kill matoi_ryuuko medium_breasts midriff navel official_art pajamas partially_colored production_art promotional_art saliva short_hair signature simple_background sketch sleeping sleepwear smile trigger_(company) white_background rating:s score:3 post #1545504](/cdn_image/preview/82/98/829872077ab273a259f99b4fe2e0156c.jpg)
From Wikipedia:
"A key frame in animation and filmmaking is a drawing that defines the starting and ending points of any smooth transition. The drawings 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 viewer 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 do not create the illusion of movement, the remaining frames are filled with inbetweens."
In other words, it is raw still shot that was used directly for the producing the animation. This covers parts, or even all of, the draft sketch to test out plot flow, pre-color scenes, cel that was used for the animation, or complete film still. It is primarily used to complete the animation, since key frames themselves do not show any movement; it is important however, if not most important in developing animations.
It is not necessarily same thing as screencap, which captures the final material in presented medium. Key frames are generally strictly internal-use only.
See also official art, production art.