late 14c., reversioun, a legal word used in reference to the return of an estate to the heirs of a grantor on the expiration of the grant, from Old French reversion and directly from Latin reversionem (nominative reversio) "act of turning back," noun of action from past-participle stem of revertere (see revert). From early 15c. as "a return to a place."
reversion has various senses, chiefly legal or biological .... It suffices to day that they all correspond to the verb revert, & not to the verb reverse, whose noun is reversal. [Fowler]