Kill stealing (or KS, Kill Steals, and Cheap Kills) is the act of killing a target that an ally (or an enemy) was targeting. This happens commonly in nearly all game modes.
Walking into small skirmishes between allies and the enemy (or enemies versing enemies in OMA matches) is not an uncommon thing to do, whether by coincidence or by choice.
There are two victims of kill steals: the player that got killed and the player that lost the kill. The killed player often is killed because they have sustained heavy damage or were careless. The player who lost the kill often loses it because of reload, ammunition exhaustion, getting killed, taking long time to aim, etc.
Kill stealing is often overemphasized, as every player will have the tendency to engage the enemy instead of sitting behind and watching allies or other enemies get killed. Most of the times, kill stealing is done out of good will, however, incompetent players can dwell over their lost kill for a very long time. Often Elite Moderators kick players for excessive kill-stealing.
In reality there is no such thing as kill stealing as kill stealing implies you own a kill. You do not and therefore it can not be stolen as there is no ownership of a kill