means A smooth white surface, usually wall-mounted, that you write on with erasable markers and wipe clean.

from A plain compound of "white" and "board" — the deliberate opposite of the older blackboard, the dark slate that schoolrooms leaned on for over a century. As dry-erase markers replaced chalk dust in the latter half of the 20th century, the surface flipped color and kept the formula: name it by its hue and its shape, and let everyone know exactly what they're staring at.