Artwork depicting a subject that isn't from any established copyright. Basically, art that is not fanart.
Some art may depict original subjects alongside subjects from an established copyright, or a fan-made character of a specific copyright. In this case, the art should be tagged as original and with the tag for the copyright and apply the tag oc x canon.
If the artwork shows a different artist's original character, include the borrowed character tag as well.
Some works on Pixiv are occasionally tagged original by accident. Art by the character designer for an established copyright also sometimes tag their work as original — on Danbooru, these should be tagged with the corresponding copyright instead.
