n
1 [U] quality or degree of being thick 厚、密、浓、混浊等（的程度）
*4cm in thickness/a thickness of 4cm 厚4厘米[4厘米的厚度].
2 [C] layer 层
*one thickness of cotton wool and two thicknesses of felt 一层棉絮及两层毛毡.
3 [C] part (of sth) that is thick or between two opposite surfaces （某物的）较厚的或夹在两层中间的部分
*steps cut into the thickness of the wall 在墙的横断面凿出的台阶.

n
[U and C]
how thick something is
　The thickness of the walls is 5 feet.
[C]
a layer of something
thickness of
　Wrap the cake in a double thickness of foil.