Conductor

A conductor is a material where valence electrons are free to move to the conduction band. The atom's conduction band is partially filled with electrons and this permits the conduction of mobile electrons.

References

Fischer-Cripps. A.C., The Electronics Companion. Institute of Physics, 2005.
Roulston. David. J., An Introduction to the Physics of Semiconductor Devices. Oxford University Press, 1999.