Architectural and Engineering Glossary


Opening Protective

A device for protecting an opening from the passage of flame,smoke,or hot gases.

Operable Transom

A panel or glass light,above a door,which may be opened for ventilation.

Operating Pressure

The pressure indicated by a gauge in part of a system,when the system is in normal operation.


On drawings,abbr.for “opposite.” 

Optical Plummet

A device on some transits and theodolites;used to center the instrument over a point,in place of a plumb bob,which moves in a strong wind.

Opus Isodomum

Same as isodomum.

Opus Latericium,opus Lateritium

Roman masonry of brick or tiles,or of a brick or tile facing on a concrete core.

Opus Listatum

A Roman wall constructed of bricks and stone in alternate layers.

Opus Lithostrotum

Same as lithostrotum opus. 

Opus Musivum

A Roman mosaic decoration employing small cubes of colored glass or enameled work.

Opus Pseudoisodomum

In ancient Roman masonry,coursed ashlar having courses of unequal height.

Opus Quadratum

Masonry of squared stones in regular ashlar courses.

Opus Sectile

See sectile opus.

Opus Signinum

An ancient Roman construction material which was employed as flooring;consisted of tiles,broken into tiny pieces,that were mixed with mortar.

Opus Spicatum

Masonry of  the ancient Romans that is laid in a herringbone pattern. 

Opus Testaceum

An early Roman concrete masonry faced with fired brick.


A small private chapel furnished with an altar and a crucifix.

Orchestra Pit

A pit immediately in front of,or wholly or partly under,the forestage of an auditorium.

Orchestra Shell

A massive,sound reflective structure which closes off the flies and wings of a theatrical stage to form a performing area for music,or is used in an open air theater to direct sound to the audience.


Abbr.for “order.”


A law or rule adopted by a local governmental authority.


A village tavern in an early American community.


Said of a material or compound derived from vegetable or animal life.

Organic Silt

A silt with a high organic content. 

Oriental Revival

A term descriptive of  a mode of Exotic Revival architecture that is suggestive of the architecture of the Middle East and/or Far East.


A secret dungeon in the deepest parts of a medieval stronghold,having as its only entry a trapdoor through which prisoners were dropped.

Oundy Molding

See wave molding.

Out And Out

Same as overall.

Out Of Winding

Said of a member that is free from twist.


Not truly vertical,according to a plumb line.

Out-of-sequence Service

A service performed in other than the normal or natural order of succession.


Not in exact alignment (as a part which is slightly twisted).


A failure in the electric power supplied by a utility company.


In masonry,a jambstone which is laid as a stretcher and cut to take a frame.


Bonded,or forming a bond,along the face of a wall;composed largely or entirely of stretchers.


A building subsidiary to,but separate from,a main house or building.


That portion of a rock formation or stratum that breaks the surface of the ground. 

Outer Hearth

See front hearth.

Outer Lining

Same as outside casing.

Outer String

The string at the outer and exposed edge of a stair,away from the wall.


The place of ultimate deposit of drainage or sewage waters.

Outfall Sewer

A sewer that receives the sewage from a sewage collection system and carries it to the point of final discharge or treatment.It is usually the largest sewer in the system.

Outline Lighting

An arrangement of incandescent lamps or gaseous tubes which outlines and emphasizes certain features such as the shape of a building or the decoration of a window.

Outline Specification

A preparatory specification,not necessarily complete but containing sufficient detail to serve as the basis for a contract specification;usually included with design development documents or schematic design documents.


1.Same as outrigger.2.A covering over a doorway or opening in the face of a build- ing to provide a small degree of shelter;also see hanging gable.


A beam at the ridge of a roof that extends beyond the end wall of the building to serve as a support for hoisting tackle or the like;also called an outlooker or lookout.

Outrigger Scaffold

A scaffold supported by brackets fastened to the wall of a building.

Outrigger Shore,horsing

A temporary bracket to support a projecting feature.


The extension of a building under a lean to roof.


Same as outshot.