Formal:
* Tableau blanc: This is the most direct translation and the most common term.
* Panneau blanc: This literally means "white panel" and can also be used.
Informal:
* Ardoise blanche: This literally means "white slate" and is more commonly used for smaller, handheld whiteboards.
Other options:
* Tableau effaçable à sec: This literally means "dry-erase board" and is a more technical term.
The best option will depend on the specific situation.