a conductor
A conductor is a material that lets electric charge move through it freely, so it carries current with very little push. Metals are the everyday example: copper wiring, aluminum power lines, the gold on a circuit board. The reason is a picture worth remembering: in a metal the outer valence electrons are not tied to any one atom. They pool together into a shared 'sea' of electrons that can drift through the whole solid, the way marbles roll freely across a tray, while the positive ion cores stay put.
In band-theory language, a conductor has its highest occupied energy band only partly filled, OR it has a filled valence band that overlaps the empty conduction band with no gap in between. Either way there are always empty energy states sitting just above the filled ones, so the tiniest electric field can nudge electrons into motion. That is why metals have very high conductivity, roughly 10^7 S/m and above, at the top of the scale. Their carrier count n is enormous, on the order of 10^28 to 10^29 free electrons per cubic meter.
A crucial and often-surprising fact: in a conductor resistivity RISES as you heat it, and RISES when you add impurities or defects. This is the opposite of a semiconductor. The carriers are already there in abundance, so extra heat does not free up more of them; instead the hotter, jiggling atoms scatter the drifting electrons more often, and each foreign atom or grain boundary is one more obstacle in the way. So the purest, coldest, most defect-free metal conducts best. This is why alloying copper (say into brass) to gain strength always costs some conductivity, a real design trade-off.
Take a copper wire from a freezer at -30 degrees C and a hot copper wire near 100 degrees C: the hot one has noticeably higher resistivity, because its atoms vibrate harder and scatter the electron sea more. Now compare pure copper with brass (copper plus 30 percent zinc): the brass is stronger and prettier but conducts only about a quarter as well, because each zinc atom is an extra scattering site.
Conductors get worse at conducting when heated or alloyed — the opposite of semiconductors.
Not every conductor is a metal — graphite, salty water, and molten salts conduct too, some by ions rather than electrons. But when we say 'a conductor' in electronics we almost always mean a metal, whose electron sea gives conductivity that barely depends on how hard you push.