What’s the difference between regime and regimen? In one sense, there is none, but in their most common connotations, one letter makes a lot of difference.
Both terms stem ultimately from Latin regere, meaning “to direct.” They are related to other words starting with reg-, such as regular, regulate, and regulation. In addition, they are akin to other words referring to direction, including reign, right, and rule and ruler, as well as the element rect in, for example, direct and rectitude. The Hindi word raja, the German term reich, and the Latin form rex are also part of this regal family tree.
Regime, borrowed from French (and sometimes, as in French, written in English as régime), can refer to a regular pattern or behavior of a natural phenomenon, but more often it denotes a form of management or rule, or a government or the duration of its rule. In this last sense, it generally has a negative connotation, implying a repressive, totalitarian government.
The phrase “ancien régime,” sometimes styled with initial capital letters, refers to the sociopolitical system in France during the several centuries preceding that country’s revolution in 1789 and, coined by revolutionaries, was a derogatory dismissal of the outmoded model of government. (However, to others it conveyed a nostalgia for a time when aristocrats, not the bourgeoisie, set cultural standards.) The generic term, as well as the Anglicized form “ancient regime” or the translation “old regime,” refers to any such ineffectual and/or corrupt government or management.
Regimen was borrowed directly from Latin into Middle English to refer to a direction, a set of rules, or a position of authority. It is, in one sense, synonymous with regime to denote rule but is now rarely used for that meaning; more often it pertains to a training system, as for athletes, or a plan intended to improve one’s health.
One other word closely related to regime and regimen is regiment. It, too, once referred to rule, but now its primary sense as a noun is a reference to a military unit that originally numbered about a thousand soldiers but in modern armies varies widely according to a unit’s particular function. The term is also used as a verb synonymous with organize, in the sense that a regiment is neatly arrayed in marching and inspection formations. However, the connotation of the verb form is often negative, implying excessive control.