Depth of Electromagnetic Penetration
Skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor, and decreases with greater depths in the conductor. The electric current flows mainly at the "skin" of the conductor, between the outer surface and a level called the skin depth. The skin effect causes the effective resistance of the conductor to increase at higher frequencies where the skin depth is smaller, thus reducing the effective cross-section of the conductor.
From: Wikipedia. See more at https://en.wikipedia.org/wiki/Skin_effect
Skin Effect Depths in Aluminum, Copper and Steel
Source: Telecommunications Distribution Methods Manual, 12th Edition; BICSI; https://www.bicsi.org/default.aspx