If you look at Wikipedia you will find general reflections on the term like this.
Collateral damage is a general term for unintentional deaths, injuries, or other damage inflicted incidentally on an intended target. In military terminology, it is frequently used where non-combatants are unintentionally killed or wounded and/or non-combatant property damaged as result of the attack on legitimate military targets. The unintentional destruction of friendly targets is friendly fire.
Critics of the term see it as a euphemism that dehumanized non-combatants killed or injured during military operations, used to reduce the perception of culpability of military leadership in failing to prevent non-combatant casualties.
Western Forces and most notably the United States are regularly criticised for collateral damage. Médecins Sans Frontières (Doctors Without Borders) ran a hospital in Afghanistan bombed by the US on October 3 this year killing 30 and wounding a further 40 doctors and staff and patients. This it seems was an accident of war as the intended target was a building that over by the Taliban several hundred meters away. The findings of the investigation concluded that this was human error and amplified by a failure of communications.
There is no doubt that this is a significant tragedy, and way outside of outcomes that we consider acceptable. Sadly, of course one must note that this is hardly a one-off. However one also notes that there is a practice in some areas of locating military targets within areas dense with unacceptable targets such as schools and hospitals.
The attack in Paris highlights another aspect of this issue. There was no collateral damage in Paris. The people struck were the intended random targets, and effectively the intention was to strike fear into us all. It seems that there is a completely different rule book, even though the result is essentially killed and injured non-combatant civilians. The ongoing problem that we face in terms of terrorism in general is that the innocent are the target. The innocent occupy soft targets.
There is no doubt that part of the purpose of terrorism is publicity, and one can not imagine what it would have cost for 24 hours of global live streaming advertising for the organisation, however the media seems convinced that the contract with the terrorists is if you can make it bad enough, we will give you the publicity.
These are challenging times. Be careful as we assess media reports, for sadly insightful and thought through analysis is thin on the ground, and whilst objectivity may well be a subjective possibility, it is still a reasonable goal that many journalists seem to have given up on.