c. 1400, "a ruler," from the adjective regent "ruling, governing" (late 14c., now archaic), from Medieval Latin regentem (nominative regens), from Latin regens "ruler, governor," noun use of present participle of regere "to rule, direct" (from PIE root *reg- "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule").
Meaning "one who rules during the minority or absence of a sovereign" is from early 15c., as an alternative to king not implying legitimacy or permanence of rule. The Latin word for this was interrex (plural interreges). Sense of "university faculty member" is attested from late 14c. and preserves the original meaning.
I shall calle unto me my counceyle of my moste trusty knyghtes and deukes and regeaunte kynges and erlys and barowns. [Malory, late 15c.]