Learner's definition of THICKNESS
1 a : the distance between the top and bottom or front and back surfaces of something : a measurement of how thick something is
b [noncount] : the quality of being thick
the length, circumference, and thickness of the log
Plywood is available in various thicknesses.
Cooking time will vary depending on the thickness of the steak/chops.
2 [count] : a layer or sheet of some material — usually + of