Baseboard

A board or molding that covers the joint where the wall meets the floor, running along the bottom of interior walls.

A straightforward American compound from 'base' + 'board,' first recorded in the 1840s as American house construction standardized. However, the concept traces back to ancient Greek and Roman architecture, where they used marble or stone 'bases' to protect wall bottoms from damage. The British call this 'skirting board' (it 'skirts' around the room), while Americans chose the more literal 'base board' — it's the board at the base.