In classical and quantum mechanics, the geometric phase is a phase difference acquired over the course of a cycle, when a system is subjected to cyclic adiabatic processes, which results from the geometrical properties of the parameter space of the Hamiltonian. The phenomenon was independently discovered by S. Pancharatnam (1956) in classical optics and by H. C. Longuet-Higgins (1958) in molecular physics; it was generalized by Michael Berry in (1984). It is also known as the Pancharatnam–Berry phase, Pancharatnam phase, or Berry phase. In classical mechanics, the geometric phase is known as the Hannay angle.

It can be seen in the conical intersection of potential energy surfaces and in the Aharonov–Bohm effect. The geometric phase around the conical intersection involving the ground electronic state of the C₆H₃F₃⁺ molecular ion is discussed on pages 385–386 of the textbook by Bunker and Jensen. In the case of the Aharonov–Bohm effect, the adiabatic parameter is the magnetic field enclosed by two interference paths, and it is cyclic in the sense that these two paths form a loop. In the case of the conical intersection, the adiabatic parameters are the molecular coordinates. Apart from quantum mechanics, it arises in a variety of other wave systems, such as classical optics. As a rule of thumb, it can occur whenever there are at least two parameters characterizing a wave in the vicinity of some sort of singularity or hole in the topology; two parameters are required because either the set of nonsingular states will not be simply connected, or there will be a nonzero holonomy.

Waves are characterized by amplitude and phase, and may vary as a function of those parameters. The geometric phase occurs when both parameters are changed simultaneously but very slowly (adiabatically), and eventually brought back to the initial configuration. In quantum mechanics, this could involve rotations but also translations of particles, which are apparently undone at the end. One might expect that the waves in the system return to the initial state, as characterized by the amplitudes and phases (and accounting for the passage of time). However, if the parameter excursions correspond to a loop instead of a self-retracing back-and-forth variation, then it is possible that the initial and final states differ in their phases. This phase difference is the geometric phase, and its occurrence typically indicates that the system's parameter dependence is singular (its state is undefined) for some combination of parameters.

To measure the geometric phase in a wave system, an interference experiment is required. The Foucault pendulum is an example from classical mechanics that is sometimes used to illustrate the geometric phase.

In a quantum system at the n-th eigenstate, an adiabatic evolution of the Hamiltonian sees the system remain in the n-th eigenstate of the Hamiltonian, while also obtaining a phase factor. The phase obtained has a contribution from the state's time evolution and another from the variation of the eigenstate with the changing Hamiltonian. The second term corresponds to the Berry phase, and for non-cyclical variations of the Hamiltonian it can be made to vanish by a different choice of the phase associated with the eigenstates of the Hamiltonian at each point in the evolution.

However, if the variation is cyclical, the Berry phase cannot be cancelled; it is invariant and becomes an observable property of the system. By reviewing the proof of the adiabatic theorem given by Max Born and Vladimir Fock, in Zeitschrift für Physik 51, 165 (1928), we could characterize the whole change of the adiabatic process into a phase term. Under the adiabatic approximation, the coefficient of the n-th eigenstate under the adiabatic process is given by $${\displaystyle C_{n}(t)=C_{n}(0)\exp \left[-\int _{0}^{t}\langle \psi _{n}(t')|{\dot {\psi }}_{n}(t')\rangle \,dt'\right]=C_{n}(0)e^{i\gamma _{n}(t)},}$$ where ${\displaystyle \gamma _{n}(t)}$ is Berry's phase with respect to parameter t. Changing the variable t into generalized parameters ${\displaystyle {\bf {R}}(t),\partial /\partial t\rightarrow -i\partial /\partial {\bf {R}}(t),}$ we can rewrite Berry's phase as $${\displaystyle \gamma _{n}[C]=i\oint _{C}\langle n({\bf {R}}(t))|{\bf {\nabla }}_{\bf {R}}|n({\bf {R}},t\rangle d{\bf {R}},}$$ where ${\displaystyle R}$ parametrizes the cyclic adiabatic process. Note that the normalization of ${\displaystyle |n,t\rangle }$ implies that the integrand is imaginary, so that ${\displaystyle \gamma _{n}[C]}$ is real. Then there is a closed path ${\displaystyle C}$ in the appropriate parameter space. The geometric phase along the closed path ${\displaystyle C}$ can also be calculated by integrating the Berry curvature over the surface enclosed by ${\displaystyle C}$.

One of the easiest examples is the Foucault pendulum. An easy explanation in terms of geometric phases is given by Wilczek and Shapere:

How does the pendulum precess when it is taken around a general path C? For transport along the equator, the pendulum will not precess. [...] Now if C is made up of geodesic segments, the precession will all come from the angles where the segments of the geodesics meet; the total precession is equal to the net deficit angle which in turn equals the solid angle enclosed by C modulo 2π. Finally, we can approximate any loop by a sequence of geodesic segments, so the most general result (on or off the surface of the sphere) is that the net precession is equal to the enclosed solid angle.