the killing of civilians in a military attack.
any damage incidental to an activity.
Origin of collateral damage
First recorded in 1985–90
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for collateral damage
military unintentional damage to civil property and civilian casualties, caused by military operations
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012