= Decoherence time
A <decoherence time> is a characteristic timescale for loss of specified off-diagonal coherence in a <density matrix>. In a simple qubit dephasing model, $\rho_{01}(t)=\rho_{01}(0)e^{-t/T_2}$, so $T_2$ is the time for that coherence magnitude to fall by a factor $e$.
In <Ramsey interferometry>, two separated pulses convert accumulated phase into an interference signal; scanning the pulse separation measures the decay of its contrast. The inferred timescale depends on the basis, environment and pulse sequence. It differs from the energy-relaxation time, and nonexponential decay may require a stated threshold or fit convention rather than a unique decay constant.
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