An emergency exit in a structure is a special exit for emergencies such as a fire: the combined use of regular and special exits allows for faster evacuation, while it also provides an alternative if the route to the regular exit is blocked by fire, etc.
It is usually a strategically located (e.g. in a stairwell, hallway, or other likely place) outward opening door with a crash bar on it and with exit signs leading to it. The name is an obvious reference to when they are frequently used, however a fire exit can also be a main doorway in or out. A fire escape is a special kind of emergency exit, mounted to the outside of a building.
Local building codes will often dictate the number of fire exits required for a building of a given size. This may include specifying the number of stairs. For any building bigger than a private house, modern codes invariably specify at least two sets of stairs. Furthermore, such stairs must be completely separate from each other. Some architects meet this requirement by housing two stairs in a "double helix" configuration where two stairs occupy the same floor space, intertwined. It may make no functional sense to have two stairs so close to each other, but it meets the requirements of the building codes.
Knowing where the emergency exits are in buildings you frequent can save your life. Some buildings, such as schools, have fire drills to practice using emergency exits. Many disasters could have been prevented if people had known where fire escapes were, and if emergency exits had not been blocked. For example, in the September 11, 2001 attacks on the World Trade Center, some of the emergency exits inside the building were inaccessible, while others were locked. In the Stardust Disaster and the 2006 Moscow hospital fire the emergency exits were locked and most windows barred shut. In the case of the Station Nightclub, the premises was over capacity the night fire broke out, the front exit was not designed well (right outside the door, the concrete approach split 90 degrees and a railing ran along the edge, and an emergency exit swung inward, not outward as code requires).
In many countries, it is required that all new commercial buildings include well-marked emergency exits. Older buildings must be retrofitted with fire escapes. In countries where emergency exits are not standard, fires will often result in a much greater loss of life.
Well-designed emergency exit signs are necessary for emergency exits to be effective. In the United States fire escape signs usually display the word "EXIT" in large, well-lit, green or red letters. Exit signs can cost $15 to $1500 dollars depending on specs. Sometimes an arrow is displayed as well. In the European Union, emergency exit signs use pictorial symbols to convey their meaning. Some signs are self-illuminated, using tritium powered traser light sources.
The number and type of exits on an aircraft is regulated through strict rules within the industry, and is based on whether the aircraft is single or twin-aisled; the maximum passenger load; and the maximum distance from a seat to an exit. The result (via model and practical testing) is that a commercial jet will be able to evacuate its designed maximum occupancy of passengers within 90 seconds even if half of the available exits are blocked.
Any aircraft where the emergency exit door sill height is above that which would make unaided escape possible is fitted with an automatic inflatable evacuation slide.