![2girls armor armored_dress ass attack back back-seamed_legwear bakuzan bare_back battle boots breasts clash colorized commentary duel elbow_gloves gloves highres honnouji_gakuen junketsu key_frame kill_la_kill kiryuuin_satsuki large_breasts long_hair matoi_ryuuko multiple_girls official_art panties production_art revealing_clothes school_uniform scissor_blade scissors screencap seamed_legwear senketsu serafuku shirt short_hair skirt student sword thigh_boots thighhighs thong trigger_(company) underwear uniform very_long_hair weapon white_background white_gloves white_legwear rating:q score:10 post #1530726](/cdn_image/preview/8f/e4/8fe4740c44b8ca7374f4b883fe9a180d.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.