Wearing clothing traditionally reserved for the opposite gender.
This is distinct from otoko no ko, which specifically refers to a male convincingly dressed in female clothing. The crossdressing tag also handles both females in male clothing, and males in female clothing who aren't fooling anyone. This tag should not be applied to futanari.
The kanji for crossdressing on pixiv is: 女装 (males in female clothing) or 男装 (females in male clothing).