I don't quite know how to define this differently, as I can't think of the proper words. The problem with saying "parallel to the ground" is that any level (the instrument, typically with a bubble in it) placed parallel to the ground if said ground is a slope (for example, a hillside) will tell you it is not level. In other words, say a table (with a flat top) is placed with its top surface level, and a (spherical) marble is placed in the middle of the top. The marble will remain stationary. If the same tabletop is placed parallel with the side of a hill, and a marble placed upon it, the marble will roll off the table in the direction of downhill. So being parallel to the ground is not necessarily a good definition of "level."
love the word