What is the meaning of Level?

A line or surface to which, at every point, a vertical or plumb line is perpendicular; a line or surface which is everywhere parallel to the surface of still water; -- this is the true level, and is a curve or surface in which all points are equally distant from the center of the earth, or rather would be so if the earth were an exact sphere.

Source: Webster’s Revised Unabridged Dictionary (1913), via OPTED