Which term is used to describe a person who is lying in a horizontal position?

The term "recumbent" specifically refers to a person who is lying down in a horizontal position, whether on their back, side, or stomach. This term is often used in medical contexts to describe the body alignment of a patient when they are not sitting or standing.

In contrast, "upright" and "vertical" refer to a position where the body is in a more or less straight and vertical alignment, which is the opposite of lying down. The term "prone" describes a specific lying position where the individual is face down, which, while related to lying down, is not as general as "recumbent". Thus, "recumbent" accurately captures the broad meaning of a person being in a horizontal position, making it the correct choice.