What tool is used to check whether a surface is perfectly flat (horizontal)?

A level (spirit level or bubble level) contains a sealed vial of liquid with a trapped air bubble. When the surface being measured is exactly horizontal, the bubble centers between the two reference lines. Various types: torpedo level (small, used in tight spaces); 4-foot level (standard for construction); laser level (projects a horizontal laser line across a room). The plumb bob checks vertical alignment (plumb), not horizontal.