Which term describes the position of lying face down and flat?

The term that accurately describes the position of lying face down and flat is "prone." In this position, the individual is lying on their stomach with their back facing upward. This term is commonly used in medical and anatomical contexts to specify the orientation of the body.

While "supine" refers to lying face up, and "recumbent" generally means lying down but does not specify the orientation (can be face up or down), "prone" distinctly identifies the face-down position. "Vertical," on the other hand, indicates an upright position and does not relate to lying down at all. Therefore, "prone" is the precise term used to describe the position of lying flat on the stomach.