Artwork depicting a subject that isn't from any established copyright. Basically, art that is not fanart.
Make sure to confirm that the art really is original before you add this tag. Google character names or search them on animecharactersdatabase.com www.animecharactersdatabase.com/
Some art may depict original subjects alongside subjects from an established copyright. In this case, the art should be tagged as original and with the tag for the copyright.
If the artwork shows a different artist's original character, include the borrowed_character tag as well.