In simple terms, the edge of something is the place or line where it stops or the part of it that is furthest from the middle.
In architecture, the use of this term is important at many levels, from the behavioural to the technical. It can refer to the interface between the natural and built environment, the place where social territories meet, the boundary between public and private, the edge of a space, and where different building materials come together. This boundary, or edge, is seen as a zone of transition that incorporates in various degrees aspects of different conditions and becomes, in its own right, another condition altogether, an ‘occupiable’ territory providing the key to the transition, connection and dialogue between different areas.
From a design point of view, the edge condition requires special attention because it often mediates between very different social and physical conditions, generating complex and often competing priorities.
In the natural world, it is a place of tension, of intensification and often of conflict. The edge between two biographical regions, between forest and field or water and land, for example, is a place of heightened activity, of a concentration of numbers of species and individuals. And, hence, of interaction, both beneficial and harmful. These natural edges are also the preferred locations of human habitation, for practical reasons and because their inherent variety and contrast make them especially appealing from a sensory and intellectual perspective. 
The aesthetic achievement of a piece of architecture may depend greatly on how well the architect resolves the various edge conditions. Case in point, wall corners and the edges of window and door openings may receive special architectural treatment in acknowledgment of their importance from both a visual and social perspective. Until the mid-nineteenth century, building walls were typically solid, opaque, and often load-bearing. In the nineteenth century, the development of the steel frame led to fundamental changes in building form and construction. Since walls were no longer primary structural elements, they could be lighter and more open. Modern architecture like that of the Bauhaus used large expanses of glass to create “a space of evenly distributed brightness,” suggesting a condition of complete equality and interconnection made possible by industrialization and mechanization. It hoped to create conditions of health, hygiene, and universality not merely in terms of physical form but in terms of social reality as well.
An example of an urban building that utilizes a kind of layered approach to edge condition is the Caltrans Headquarters located at 100 Main Street in downtown Los Angeles, designed by Morphosis and completed in 2004. Caltrans embraces the dispersed, car-oriented urban condition of Los Angeles by conceptualising a building in a sort of spatial sequences, materiality, and green technology which create conditions of overlap and interconnection.
From the street, the building is approached via an exterior corner plaza that becomes gradually more enclosed as one approaches the building entrance. It leads diagonally across the site to an exterior “urban lobby” surrounded by a neon sculpture that extends into the plaza. This urban lobby reaches back into the mass of the building, enclosed on three sides by the neon-covered walls and roofed by the building mass. It causes the focus of the complex to be a partly interior, partly exterior space that is publicly accessible and constantly changing. By bringing the public into the very heart of a civic building, rendering ambiguous the transition from public to private, and blurring the distinction between interior and exterior, it creates a place defined by relationship and interconnection rather than definition and separation.
Today’s cities are characterized by multiperspectivism, an infinite number of perspectives that are valid because of their differences, and the seemingly contradictory simultaneous erasure of boundaries wherein the local becomes universal and differences dissolve while they are being strengthened. This urban condition suggests the both/and condition of transparency described by Colin Rowe and Robert Slutzky as “a simultaneous perception of different spatial locations” and the ability to “interpenetrate without an optical destruction of each other” that is not typical of the clearly defined but rather of the clearly ambiguous.