In magnetic resonance spectroscopy the longitudinal relaxation time, T 1, is associated with spin-lattice relaxation, and the transverse relaxation time, T 2, with spin-spin relaxation. The definitions are: dMz/dt = (-Mz - Mz,e)/T1 and dMx/dt = -Mx/T2, where M z and M x are the components of magnetization parallel and perpendicular to the static field B and the subscript e denotes the equilibrium value.
Source: Green Book, 2nd ed., p. 25 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
In a chemical reaction, the time, τ, in which a concentration perturbation falls to 1/e of its initial value.
Source: Green Book, 2nd ed., p. 55 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
See also: PAC, 1996, 68, 149. (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 185 [Terms] [Paper]