In physics, the spin–spin relaxation is the mechanism by which M_xy, the transverse component of the magnetization vector, exponentially decays towards its equilibrium value in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). It is characterized by the spin–spin relaxation time, known as T₂, a time constant characterizing the signal decay. It is named in contrast to T₁, the spin–lattice relaxation time. It is the time it takes for the magnetic resonance signal to irreversibly decay to 37% (1/e) of its initial value after its generation by tipping the longitudinal magnetization towards the magnetic transverse plane. Hence the relation

${\displaystyle M_{xy}(t)=M_{xy}(0)e^{-t/T_{2}}\,}$.

T₂ relaxation generally proceeds more rapidly than T₁ recovery, and different samples and different biological tissues have different T₂. For example, fluids have the longest T₂ (on the order of seconds for protons), and water based tissues are in the 40–200 ms range, while fat based tissues are in the 10–100 ms range. Amorphous solids have T₂ in the range of milliseconds, while the transverse magnetization of crystalline samples decays in around 1/20 ms.