The コスプレ (cosplay) tag is an abbreviation for "costume play". Cosplay is the act of dressing up as a fictional character.
This tag is meant to be used specifically for instances where a character (either an original character or copyrighted) is cosplaying as a specific character from a different work of fiction. It should not be used for instances where a character is in a generic uniform (such as a school uniform, police uniform, nurse uniform, etc.) even if the character does not normally wear that type of clothing. For those instances, please use the appropriate tag for the type of clothes the character is wearing rather than the cosplay tag.
Additionally, the tag should not be used when characters from the same piece of fiction have switched clothing with each other or when a character is disguised as another character from the same fictional setting. For that situation use the 衣装チェンジ (Changed Clothing) tag.
Also, the tag should not be used to when characters are wearing clothing of an original design by the artist, even if the character in question doesn't normally wear clothes like that. The tag should also not be used when characters from fiction are wearing generic casual clothes. For that situation use the 非公式服 (informal clothes) or 私服 (plain clothes) tags.