relaxation time
  1. In magnetic @R05326@ @S05848@ the longitudinal @R05285@ time, \(T_{1}\), is associated with spin-lattice @R05285@, and the transverse @R05285@ time, \(T_{2}\), with spin-spin @R05285@. The definitions are: \(\frac{\text{d}M_{z}}{\text{d}t} = -\frac{M_{z} - M_{z,\text{e}}}{T_{1}}\) and \(\frac{\text{d}M_{x}}{\text{d}t} = -\frac{M_{x}}{T_{2}}\), 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.
    Green Book, 2nd ed., p. 25 [Terms] [Book]
  2. In a chemical reaction, the time, \(\tau \), in which a concentration perturbation falls to \(\frac{1}{\text{e}}\) of its initial value.
    Green Book, 2nd ed., p. 55 [Terms] [Book]
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]