thick·ness / ˈ[unvoicedth]iknis/ • n. 1. the distance between opposite sides of something: the gateway is several feet in thickness | paving slabs can be obtained in varying thicknesses. ∎  the quality of being broad or deep: the immense thickness of the walls. ∎  a layer of a specified material: the framework has to support two thicknesses of plasterboard. ∎  [in sing.] a broad or deep part of a specified thing: the beams were set into the thickness of the wall.2. the quality of being dense: he gave his eyes time to adjust to the thickness of the fog. ∎  the state or quality of being made up of many closely packed parts: the thickness of his hair.

thickness The difference in height in the atmosphere between particular pressure levels, e.g. between 1000 and 500 millibars. Isolines of a given interval on charts are termed ‘thickness lines’.