Alternate costumes of official origin, whether it be from the creators themselves, or from second-parties involved in the given copyright. Generally a costume other than the standard one the character appears in.
These can often times appear in manga/anime (beach episodes, as one example), video games (whether as unlockables or as skins to be bought in gacha games), and on special events. Figures which can have such costumes include Virtual YouTubers.
Tag is best used with fan art since official art introduced such alternate costumes to everyone's consciousness in the first place.