Origins
The word 'regime' entered English in the fifteenth century from French 'régime,' which descended from Latin 'regimen,' meaning 'guidance,' 'rule,' 'government,' or 'a system of management.' The Latin noun derives from 'regere' (to guide, to rule, to make straight), from PIE *h₃reǵ- (to move in a straight line, to direct, to rule).
The word has three interconnected but distinguishable modern senses. First, political: a 'regime' is a system of government, often with authoritarian or pejorative connotations. 'The military regime,' 'the totalitarian regime,' 'regime change' — in political discourse, 'regime' often implies illegitimacy or authoritarianism. This is a semantic development that occurred in English (and French) during the nineteenth and twentieth centuries; the word originally carried no negative connotation. The French phrase 'Ancien Régime' (Old Regime), referring to the pre-revolutionary French monarchy, helped establish the association between 'regime' and systems ripe for overthrow.
Second, systematic: a 'regime' is any ordered system of doing things. A 'fitness regime,' a 'beauty regime,' a 'training regime' — in these contexts, the word means a structured, regular program. This sense is closer to the Latin original, where 'regimen' meant guidance or systematic management. (Note that 'regimen' exists as a separate English word, a doublet of 'regime,' with this same meaning. In careful usage, 'regimen' is preferred for personal programs like diet and exercise, while 'regime' is reserved for political systems, but many speakers use them interchangeably.)
Scientific Usage
Third, scientific: in hydrology, ecology, and other sciences, 'regime' describes a characteristic pattern or set of conditions. A 'flow regime' describes the pattern of water movement in a river. A 'fire regime' describes the historical pattern of fires in an ecosystem. A 'thermal regime' describes the pattern of temperature variation. In this technical sense, 'regime' means 'the prevailing system of conditions.'
The Latin root 'regere' connects 'regime' to an enormous English vocabulary. 'Regal' (kingly), 'reign' (the period of ruling), 'regent' (one who rules), 'region' (a ruled area), 'regular' (according to the rule), 'regulate' (to subject to rule), 'regiment' (a ruled military unit), 'correct' (set thoroughly straight), 'erect' (set straight upward), 'direct' (set straight toward), 'rectangle' (with right angles), and 'right' itself (from the Germanic cognate) all descend from *h₃reǵ-.
The triplet 'regime / regimen / regiment' illustrates how the same Latin word can enter English through multiple channels and acquire different specializations. 'Regime' came through French with a political emphasis. 'Regimen' was borrowed directly from Latin with a medical and systematic emphasis (a doctor's regimen for a patient). 'Regiment' added the Latin suffix '-mentum' and specialized as a military term for a unit under strict command and discipline. All three preserve the core concept of organized guidance — a system that keeps things moving in a straight line.