The Berry phase is the geometric phase acquired by a quantum system when its parameters are changed slowly around a closed loop.
Michael Berry described the effect in 1984. After the system returns to its original physical conditions, its wavefunction can differ by a phase determined by the path taken, not only by the elapsed time or energy.
The phenomenon is called geometric because it depends on the geometry of the route through parameter space. It appears in molecular physics, optics, condensed matter, and quantum computation. In solids, related Berry-curvature effects help explain anomalous Hall transport and some topological phenomena.
Berry phase is distinct from an ordinary dynamical phase, which depends on energy and time. The Aharonov–Bohm effect is another important example of geometric phase behavior, but it is not the same named effect.