(T2) The return of the transverse magnetization to its equilibrium value, zero.

Return of longitudinal magnetization to its equilibrium value after excitation;; requires exchange of energy between the nuclear spins and the lattice. See also T1 Time.

(T1) The return of the longitudinal magnetization to its equilibrium value along the +z axis.

The spin spin relaxation time is the time the spins need to dephase in the transverse plane.
See T2 Time, and Transverse Relaxation Time.

The T2 time constant, which determines the rate at which excited protons reach equilibrium, or go out of phase with each other. A measure of the time taken for spinning protons to lose phase coherence among the nuclei spinning perpendicular to the main field due to interaction between spins, resulting in a reduction in the transverse magnetization. The transverse magnetization value will drop from maximum to a value of about 37% of its original value in a time of T2.