On danbooru the term "oekaki" refers to drawings made with very simple tools, usually imageboard/bbs-embedded drawing software, with very sharp and thin lines. Often overlaps with sketch.
Oekaki (お絵描き) is Japanese for "scribble" or "doodle". On the Internet, an Oekaki is a message board system that revolves around computer art created by a local art program. Oekaki artists often post their art to other websites, such as personal pages or free gallery websites, in addition to the original board.