A core is understood to be a single wire with an insulating sheath of a transmission medium, which essentially determines the attenuation behavior of the conductor through its conductor resistance. To achieve the lowest possible attenuation values, the material should have a high conductivity.
In practice, a core usually consists of copper in bare, tinned, nickel-plated or silver-plated form. If the transmission medium has two wires that belong together, it is referred to as a pair of cores. The diameter of cores is specified in the American wire gauge AWG, in Germany also in mm. Since in practice not any number of conductor cross-sections can be used, conductor diameters of 0.4 mm to 0.7 mm have been agreed upon.
In the case of optical fibers, the actual glass fiber consisting of core and cladding glass together with the surrounding primary and secondary coating and the cable sheath is referred to as the core.