During nuclear magnetic resonance observations, spin–lattice relaxation is the mechanism by which the longitudinal component of the total nuclear magnetic moment vector (parallel to the constant magnetic field) exponentially relaxes from a higher energy, non-equilibrium state to thermodynamic equilibrium with its surroundings (the "lattice").  It is characterized by the spin–lattice relaxation time, a time constant known as T.
There is a different parameter, T, the spin-spin relaxation time, which concerns the exponential relaxation of the transverse component of the nuclear magnetization vector ( to the external magnetic field). Measuring the variation of T and T in different materials is the basis for some magnetic resonance imaging techniques.