Final answer:
The opposite of supine, which means lying face up, is prone, meaning lying face down.
Step-by-step explanation:
The opposite of supine, which describes a body orientation where one is lying face up, is prone. The prone position refers to lying face down. When categorizing body positions, particularly in medical contexts during physical examinations or surgical procedures, these terms are important for clear communication. Supine and prone correspond to specific body orientations and are distinct from other upright or vertical positions such as standing or sitting.