Wearing clothing traditionally reserved for the opposite sex. (e.g. Men wearing dresses, women wearing tuxedos, etc.)
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.
Futanari and newhalf are considered to be crossdressing if they are wearing men's clothing.
The kanji for crossdressing on pixiv is: 女装 (males in female clothing) or 男装 (females in male clothing).