Collateral damage is damage that is unintended or incidental to the intended outcome. The term originated in the United States military, but it has since expanded into broader use.
At least one source claims that the term "collateral damage" originated as a euphemism during the Vietnam War and can refer to friendly fire, or the killing of non-combatants and the destruction of their property. Curtis Le May used the term in describing the bombing of Japanese cities in the Second World War.
The term 'collateral damage' has also been borrowed by the computing community to refer to the denial of service to legitimate users when administrators take blanket preventative measures against some individuals who are abusing systems. For example, Realtime Blackhole Lists used to combat email spam generally block ranges of Internet Protocol (IP) addresses rather than individual IPs associated with spam, and can deny legitimate users within those ranges the ability to send email to some domains.
A related term 'collateral mortality' is also becoming prevalent, and probably derives from the term collateral damage. This has been applied to other spheres in addition of the original military context. An example is in fisheries where bycatch of species such as dolphins are called collateral mortality; i.e. they are species that die in pursuit of in the legal death of fishery targets; e.g. tuna.
- In an interview before his execution, convicted U.S. bomber (and Gulf War veteran) Timothy McVeigh referred to the deaths of 19 children killed in the government office building during the April 1995 Oklahoma City bombing as "collateral damage".
- Jamie Shea, spokesman of NATO during NATO campaign against Federal Republic of Yugoslavia, has became (in)famous for using this phrase in comments about civilian casualties killed by NATO.
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