'Regime' traces through French to Latin 'regere' (to guide straight) — governance as keeping the line.
A government, especially an authoritarian one; a system or ordered way of doing things; a prescribed course of medical treatment, diet, or exercise.
From French 'régime,' from Latin 'regimen' (guidance, rule, government), from 'regere' (to guide, to rule, to make straight). The PIE root is *h₃reǵ- (to move in a straight line, to direct, to rule), one of the most productive roots in Latin, generating words for royal authority, correct alignment, and governance. The same root gives English 'regent,' 'regal,' 'regulate,' 'rector,' and 'right' (via Germanic
The words 'regime,' 'regimen,' and 'regiment' are triplets — all from Latin 'regimen' (guidance, rule). 'Regime' came through French (emphasizing political rule), 'regimen' was borrowed directly from Latin (emphasizing a prescribed system), and 'regiment' added a suffix to denote a military unit under strict rule.