Infamy is a Monster Play article.
Infamy, for Monster Players, is the counterpart of Renown gained by Player Characters and similar to experience in how it functions.
Each time you defeat an enemy player (a Free Peoples-side Player Character -- NOT an NPC), whether solo, in a Warband or in a Raid, you earn an amount of Infamy largely based on the Rating of the defeated foe. Earned Infamy is never lost, and continues to drive you closer to ever higher Ranks.
Current Infamy appears on The War page of your Character Journal.