“A LEVEL surface is one that is everywhere orthogonal to a plumb line – meaning that any tangent to the surface forms a 90 degree angle with a plumb line at that point.”
-George Hnatiuk
Also see the collection of definitions of level by Jesse Kozlowski, a licensed geodetic surveyor: https://jessekozlowski.wordpress.com/2020/02/19/definition-of-level/
From “The Theory and Practice of Surveying; Containing All the Instructions requisite for the skillful practice of this Art.”, published 1814
This dictionary:
Has this definition:
“Elementary Surveying, An Introduction to Geomatics 13th ed”: