BCS theory is the fundamental theory explaining superconductivity in materials, formulated by John Bardeen, Leon Cooper, and Robert Schrieffer. It describes how electron pairs (Cooper pairs) form and move through a lattice without resistance.
Superconductivity: A phenomenon where a material exhibits zero electrical resistance below a certain critical temperature.
Cooper Pair: A bound pair of electrons with opposite spins and momenta that move together through a superconductor without resistance.
Critical Temperature: $T_c$, the temperature below which a material becomes superconducting.