Cooper pair

A Cooper pair is a pair of free electrons within a solid that act together as one quasiparticle and in large numbers give rise to superconductivity.
Physicists explain superconductivity by describing what happens when temperatures get cold. The thermal energy in a solid or liquid shakes the atoms so they randomly vibrate, but this gets less as the temperature drops. Electrons carry the same negative electric charge which makes them repel each other. At higher temperatures each unbound electron behaves as if it were a free particle. There is also however a very weak attraction between electrons when they are in any solid or liquid. This is due to the way bound electrons distort the spacing of the atomic structure they find themselves in. At relatively rather large distances (many hundreds of nanometers apart) and low temperatures (near absolute zero), the attractive effect and lack of thermal energy allows pairs of electrons to hang together. This is called a cooper pair and it is a quasiparticle, that is it acts as if it were a new kind of particle in its own right even though it is made up of two fundamental electrons. 
Many overlapping cooper pairs can exist in the same nanometer sized space. Since paired electrons constitute a boson the motions of all of the cooper pairs within a single superconductor synchronise and they function as if they are a single entity. This is known as the Bose–Einstein condensation. The Cooper pairs form this condensate, which is superfluid or super-conducting. Small disturbances such as scattering of electrons are forbidden in this state as long as the temperature remains low. It moves as one showing no resistance to its motion. It is hence now a superconductor.