Toward the inside relative to the outside. This word is used when an organism is in anatomic position and it is necessary to describe a feature on its body so that others can understand without actually seeing the organism where something is in reference to something else. The lungs are deep to the skin.

Also see superior, posterior, ventral, dorsal, cephalic, caudal, proximal, distal, medial, lateral, superficial, prone, supine, plane, and region.