A material that lets electricity flow easily. This is because the electrons are loosely bound to the nucleus, meaning there is little to no energy required to pull the electron away. This gives conductors a non-existent bandgap, where the valence and conduction bands are literally touching or even overlapping.
A material that conducts electrical current very well.
Electronic Devices : Conventional Current Version, 9th Edition by Thomas L. Floyd
Any material that allows the free movement of electric charges, such as electrons, to provide an electric current.
Grob’s Basic Electronics, 11th Edition by Mitchel E. Schultz