The longitudinal relaxation time or spin-lattice relaxation time (T1) describes the rate at which magnetization M returns to the thermodynamic equilibrium orientation along B0 after a rf pulse. T1 is usually given in units of seconds. T1 is correlated with the overall rotational tumbling of the molecule in solution and may be further affected by intramolecular mobility in flexible structures.
From: Wüthrich, K. (1986). NMR of Proteins and Nucleic Acids, John Wiley & Sons, New York.