A part of a surface that is lower than the rest can be said to be depressed or indented, and the area thus created can be called a depression or an indentation.
depression noun (PRESS DOWN) [ C ] a part in a surface that is
slightly lower than the rest:
There was a depression in the sand where he'd been lying.
indentation noun (HOLE) [ C ] a hole or mark on the surface of
The heels of her shoes had left indentations in the mud.
Slatter's Fundamentals of Veterinary Ophthalmology 5th Edition (2013, David Maggs, Paul Miller, Ron Ofri), page 99, gives a description, in ordinary educated language, of a technique used in animal and human medicine to measure the IOP (intra-ocular pressure) of an eyeball:
The Schiotz tonometer relies on indentation tonometry. In this method,
a standard force is applied with a metal rod to a topically
anesthetized cornea. The distance the rod indents the cornea is
measured and is inversely related to the LOP (i.e., the greater the
tonometer scale reading, the lower the patient's IOP). This concept is
easily understood if the eye is regarded as analogous to a
water-filled balloon. If the blunt end of a pencil is applied to the
balloon with a given force (e.g., the weight of the pencil placed
vertically), the pencil indents the surface of the balloon by a
certain distance. If the pressure in the balloon is decreased (some of
the water is let out), the tension in the rubber wall decreases, and
the same pencil resting on the balloon indents it farther. Conversely,
if the pressure in the balloon is increased, the same pencil indents
it less. The Schiotz tonometer (Figure 5-40) consists of three parts:
the plunger (analogous to the pencil), the footplate assembly (a
device to measure indentation), and the handle. A further refinement is added: The weight applied to the eye through the rod may be
varied by adding or subtracting weights (5.5 g, 7.5 g, 10 g, or 15 g).
The greater the weight applied to the eye at a given IOP, the greater
the penetration of the rod.