A pergola is most commonly an outdoor garden feature forming a shaded walkway, passageway, or sitting area of vertical posts or pillars that usually support cross-beams and a sturdy open lattice, often upon which woody vines are trained. The origin of the word is the Late Latin pergula, referring to a projecting eave.
It also may be an extension of a building or serve as protection for an open terrace or a link between pavilions. They are different from green tunnels, with a green tunnel being a type of road under a canopy of trees.
Pergolas are sometimes confused with "arbors," as the terms are used interchangeably. Generally, an "arbor" is regarded as wooden bench seats with a roof, usually enclosed by lattice panels forming a framework for climbing plants; in evangelical Christianity, brush arbor revivals occur under such structures. A pergola, on the other hand, is a much larger and more open structure. Normally, a pergola does not include integral seating.
Modern pergola structures can also include architectural or engineering structures having a pergola design, which are not used in gardens. California High-Speed Rail, for instance, uses large concrete pergolas to support high-speed rail guideways which cut over roadways or other rail tracks at shallow angles (unlike bridges or over-crossings which are usually nearly at right angles). (See the high-speed rail pergola structure picture elsewhere in the article for an illustration.)