1. The return of a system that has had a sudden disturbance to its equilibrium state. Relaxation time is the average time that it takes for the system to relax. An example of a relaxation time is the time it takes for a gas to return to the Maxwell–Boltzmann distribution after it has been suddenly disturbed from that state. Relaxation is frequently discussed in terms of a response function.
2. In magnetic resonance, relaxation from an excited state to the ground state. There are two types of relaxation in magnetic resonance, each of which is characterized by its own relaxation time. The first type is spin-lattice relaxation, in which relaxation occurs because of the interaction between the spin system and its environment (which does not have to be a lattice), and the second is spin-spin relaxation, in which relaxation occurs because of interactions between spins. It is because the two relaxation processes are physically different that the relaxation times associated with these processes are different.