The term "be vertical" refers to the position of an object or person in an upright direction, or perpendicular to a horizontal surface. Antonyms for this term include "be horizontal," which is the opposite position where an object or person lays flat on a surface. Additionally, "be inclined" is another antonym, which means to be at a slant or angle. "Be tilted" is also an antonym that refers to objects or persons leaning or tilting to one side. Lastly, "be prone" is another antonym which refers to objects or persons lying face down on a surface. Overall, these antonyms provide various positions that are opposite to being vertical.