1. naked or unclothed, as a person or the body.
2. without the usual coverings, furnishings, etc.; bare: a nude stretch of land laid waste by brush fires.
3. (of a photograph, painting, statue, etc.) being or prominently displaying a representation of the nude human figure.
4. Law. made without a consideration or other legal essential: a nude contract.
5. having the color nude.
The two nude subcategories - Formal, classical and playful, suggestive can be found here - Female Nudes and Male Nudes.
Nude stock is a great reference for people wishing to develop a better grasp on anatomy without having to guess at what legs/arms etc are doing under shirts. Thankfully we have a good gallery full of nudes