The term relaxation time is the time for some process to return
to its normal/equilibrium state (relaxation).
The term is often used for a quantification of this time in cases
where the return is characterized by some quantity falling off
exponentially.
This is the typical case for the overall relaxation that is a
composite of the relaxations of many individual items (e.g.,
molecules, atoms, or other particles).
Relaxation time is the time for that quantity to fall by a factor
of e. The return-to-normal's half-life carries the same
information but scaled to the time in which the quantity halves.