A hasty or undetailed drawing or painting often made as a preliminary study.
Traits of sketches include rough lines, basic coloring, and left over structural shapes. If a picture does not have any of the latter please refrain from tagging it a sketch.
Abbreviated on pixiv as "rkgk", which comes from the japanese word for sketch (rakugaki).
Sketches are not sufficient reasons for flagging. See howto:flag.