What is flat surface?

Answer 1

A flat surface generally refers to a horizontal surface and is referred to as the plane.

Answer 2

The word flat is an English word written and spoke by English Peoples thousands of years before engineering scientists were born and adopted that flat word used on earth. Any planet which has own gravitational force suitable for human being to walk, build house to live, to make more flat surface as road, city, further more flat surface as house flooring is known as flat surface. According to atomic theory electron, proton, neutron are considered as round cube with absolute smooth surface but has no gravitational force as a planet have for human beings to build house, road , city etc. These all scientific theories are after the birth of English Language.Stupid scientists mostly used that ancient word and applied it in a different manner without another word added or applied to it. So earth has flat surface. Similarly sea level is known as constant as it same all over the world. In civil engineering water level is also used to check flat surface on the surface of earth including flat road, building, flat flooring etc. So overall surface of earth is known as flat.