A hasty or undetailed drawing or painting often made as a preliminary study. Traits include rough lines, monochrome or basic coloring, and left over structural shapes.
Abbreviated on Pixiv as "rkgk", which comes from the Japanese word for sketch (落書き, rakugaki). Not all pictured tagged as "sketch" by the artist should be tagged as sketch on Danbooru. The picture must look like a sketch for it to apply. Translated tags have been removed from this wiki, because artists tend to use this tag in a very misleading way.
Sketches are not sufficient reasons for flagging. See howto:flag.