Kill Stealing is the practice of attacking a monster already under attack by another character not in one's party. The result is that the monster is killed by the second attacker, who gets credit for it, while the original attacker does not. This is a minor issue regarding experiance points, but may be a major issue regarding Quest targets. The latter is further amplifyed in the case of rare or difficult monsters.
Accidental Kill Steal
While this may be done accidentally, it is important for all players to be concious of thier environment and avoid Kill Stealing. Most players will forgive the occasional accidental Kill Steal, but repeated offenses are likely to get the offender reported.