Which term is used to describe the position of the body when lying on the back?

The term used to describe the position of the body when lying on the back is "supine." This position involves the individual lying flat on their back with their face and torso facing upward. The supine position is commonly used in medical settings, such as during examinations, surgeries, or treatments, as it provides easy access to the front of the body.

In contrast, the prone position refers to lying face down, which is the opposite of supine. The terms vertical and horizontal describe orientations rather than specific body positions. Vertical refers to an upright position, while horizontal typically describes a lying position but does not specify how the body is oriented. Therefore, the term "supine" accurately conveys the specific posture of lying on the back.