cel
Also called celluloids, they are plastic sheets used in traditional animation by hand-drawing and coloring images (or frames) on individual sheets, then shooting them in a sequence and then adding voices, music and SFX in post-production to create the effect of animation. This method of animation has been replaced with computer animation, so cel-animated shows are not widely seen anymore.
For Danbooru, this tag refers to an individual frame of animation being scanned and uploaded.