The word "collateral" comes from medieval Latin collateralis, from col-, "together with" + lateralis (from latus, later-, "side" ) and is otherwise mainly used as a synonym for "parallel" or "additional" in certain expressions ("collateral veins" run parallel to each other and "collateral security" means additional security to the main obligation in a contract). However, "collateral" may also sometimes mean "additional but subordinate," i.e., "secondary" ("collateral meanings of a word"), and that specific meaning of a rather obscure word in the English language seems to have been picked up and broadened by the military in the expression "collateral damage".
The USAF Intelligence Targeting Guide defines the term "[the] unintentional damage or incidental damage affecting facilities, equipment, or personnel, occurring as a result of military actions directed against targeted enemy forces or facilities. Such damage can occur to friendly, neutral, and even enemy forces". Another United States Department of Defense document uses "Unintentional or incidental injury or damage to persons or objects that would not be lawful military targets in the circumstances ruling at the time. Such damage is not unlawful so long as it is not excessive in light of the overall military advantage anticipated from the attack."
Intent is the key element in understanding the military definition as it relates to target selection and prosecution. Collateral damage is damage aside from that which was intended. Since the dawn of precision guided munitions, military "targeteers" and operations personnel are often considered to have gone to great lengths to minimize collateral damage.
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.