A monorail is a rail-based transportation system based on a single rail, which acts as its sole support and its guideway. The term is also used variously to describe the beam of the system, or the vehicles traveling on such a beam or track. The term originates from joining mono (one) and rail, from as early as 1897, possibly from German engineer Eugen Langen who called an elevated railway system with wagons suspended the Eugen Langen One-railed Suspension Tramway (Einschieniges Hängebahnsystem Eugen Langen). The transportation system is often referred to as a railway.
Colloquially, the term "monorail" is often used erroneously to describe any form of elevated rail or people mover. In fact, the term refers to the style of track, not its elevation.
Read more about Monorail: Differentiation From Other Transport Systems, Types and Technical Aspects, Gauge, Nomenclature