A regiment is a title used by some military units. The size of a regiment varies markedly, depending on the country and the arm of service.Originally the term "regiment" simply denoted a large body of men under a single leader. Historically, in the 17th century, a full-strength regiment was hypothetically a thousand men commanded by a colonel. Today, there is no set size for a unit calling itself a "regiment", which may be:Less than a battalion-equivalent, e.g.