In nuclear magnetic resonance (NMR) logging, the characteristic time for transverse relaxation. In rocks, there are three components of the transverse relaxation: surface, T_{s}; bulk, T_{b}; and diffusion relaxation, T_{d}. T_{2} is the inverse sum of each component for each fluid, as follows:
1/T_{2} = 1/T_{s} + 1/T_{b} + 1/T_{d}.
Because of the reciprocal sum, the smallest of the three types of relaxations is the most important in determining the final T_{2} for each fluid. There is not one single value of T_{2} for a rock but a wide distribution of values lying anywhere between fractions of a millisecond and several seconds. The distribution of T_{2} values is the principal output of an NMR log.