  Spintronics is an emerging paradigm with the aim to replace conventional
electronics by using electron spins as information carriers. Its utility relies
on the magnitude of the spin-relaxation, which is dominated by spin- orbit
coupling (SOC). Yet, SOC induced spin-relaxation in metals and semiconductors
is discussed for the seemingly orthogonal cases when inversion symmetry is
retained or broken by the so-called Elliott-Yafet and Dyakonov-Perel
spin-relaxation mechanisms, respectively. We unify the two theories on general
grounds for a generic two-band system containing intra- and inter-band SOC.
While the previously known limiting cases are recovered, we also identify
parameter domains when a crossover occurs between them, i.e. when an inversion
symmetry broken state evolves from a Dyakonov-Perel to an Elliott-Yafet type of
spin-relaxation and conversely for a state with inversional symmetry. This
provides an ultimate link between the two mechanisms of spin-relaxation.
