Final answer:
A person lying flat on their stomach is in the prone position, which is important in medical fields for examinations and surgeries.
Step-by-step explanation:
A person who is lying flat on his or her stomach is said to be in the prone position. This term is used to describe an orientation where the person's face and body are directed downwards. The prone and supine positions are commonly used terms in medical and health fields to describe a person's orientation when they are lying down. While a prone position refers to lying face down, a supine position means lying face up. These positions are important in various contexts, including physical examinations, surgical procedures, and discussions of proper body alignment for good posture.