The Merriam-Webster dictionary defines "collateral damage" as: "injury inflicted on something other than an intended target; specifically : civilian casualties of a military operation" 
Though the term has come into the common vernacular today, it's origins are military.
The first known use of this term by the United States was in 1968. It was originally used to describe civilian deaths caused by nuclear weapons . The term became more commonly used during the Vietnam War, especially when talking about the use of widespread, imprecise killing tactics, such as carpet bombing (bombing areas rather than specific targets ) and the use of Agent Orange, which were common during that time. .
The controversy with the term begins early. Some sources claim the term was actually used among soldiers during the Vietnam War as a euphemism to refer to the intentional killing of noncombatants . Critics of the term claim it dehumanizes deaths, and is used mostly by governments as a way to excuse wartime atrocities .
|Another way of looking at "collateral damage" |
Determining what exactly constitutes "collateral damage" and what constitutes negligence (or, in extreme cases, a war crime) can be difficult. Collateral damage isn't addressed in international law. However, the Geneva Conventions contain certain restrictions on indiscriminate attacks. Article 57 of the 1977 Addition Protocol I to the 1949 Geneva Conventions, for example, states that "constant care shall be taken to spare the civilian population, civilians, and civilian objects" during international conflicts. Article 51 of the Geneva Conventions prohibits carpet bombing, as well as attacks whose effects cannot be controlled . It's worth noting that both of these articles went into effect long before the start of the Vietnam War.
 Anthony H. Cordesman (2003). The Iraq War: Strategy, Tactics, and Military Lessons. Praeger/Greenwood. p. 266.