Revenge kill

From TF2 Wiki

Players receive a Revenge kill when they kill a Nemesis. A Revenge kill is worth double the Points you would receive for a regular kill. Revenge kills in one life as well as in your player history count towards your TF2 stats. Players receive the 'Nemesis' achievement upon collecting 5 total Revenge kills in their all-time stats.

A Revenge kill can also be achieved by getting a kill assist on your Nemesis. For example, when you are healing a Heavy and he kills your Nemesis. A Revenge kill can also occur during Humiliation and this is a highly advantageous time to get one as your Nemesis will be completely vulnerable.

A Revenge kill is accompanied by a short flourish, a message on the recent kills log and possibly a specific response rule.