Survivability is the ability to remain alive or continue to exist. The term has more specific meaning in certain contexts.
Ecological
Following disruptive forces such as flood, fire, disease, war, or climate change some species of flora, fauna, and local life forms are likely to survive more successfully than others because of consequent changes to their surrounding biophysical conditions.
Engineering
In engineering, survivability is the quantified ability of a system, subsystem, equipment, process, or procedure to continue to function during and after a natural or man-made disturbance; for example a nuclear electromagnetic pulse from the detonation of a nuclear weapon.
For a given application, survivability must be qualified by specifying the range of conditions over which the entity will survive, the minimum acceptable level or post-disturbance functionality, and the maximum acceptable downtime.
Military
In the military environment, survivability can be defined as the ability to remain mission capable after a single engagement. Engineers working in survivability are often responsible for improving four main system elements:
Detectability - the inability to avoid being aurally and visually detected as well as detected by radar (by an observer). Susceptibility - the inability to avoid being hit (by a weapon). Vulnerability - the inability to withstand the hit. Recoverability - longer-term post-hit effects, damage control, and firefighting, capability restoration, or (in extremis) escape and evacuation.
The European Survivability Workshop introduced the concept of "Mission Survivability" whilst retaining the three core areas above, either pertaining to the "survivability" of a platform through a complete mission, or the "survivability" of the mission itself (i.e. probability of mission success). Recent studies have also introduced the concept of "Force Survivability" which relates to the ability of a force rather than an individual platform to remain "mission capable".
There is no clear prioritisation of the three elements; this will depend on the characteristics and role of the platform. Some platform types, such as submarines and airplanes, minimise their susceptibility and may, to some extent, compromise in the other areas. Main Battle Tanks minimise vulnerability through the use of heavy armours. Present day surface warship designs tend to aim for a balanced combination of all three areas.
A popular term is the "survivability onion"; described as 5-8 layers: Don't be there. If you are there, don’t be seen. If you are seen, don’t be targeted/acquired. If you are targeted/acquired, don’t be hit. If you are hit, don’t be penetrated. If you are penetrated, don’t be killed.
Naval
Survivability denotes the ability of a ship and its on-board systems to remain functional and continue designated mission in a man-made hostile environment. The naval vessels are designed to operate in a man-made hostile environment, and therefore the survivability is a vital feature required from them. The naval vessel’s survivability is a complicated subject affecting the whole life cycle of the vessel, and should be considered from the initial design phase of every war ship.