A moniker is a nickname. Basketball player Charles Barkley had the moniker "The Round Mound of Rebound."
A moniker is a nickname or pet name for a person. People who are dating or friends often have monikers like "Sweetie" and "Schmoopie." Some monikers are shortened versions of your name, like "Ed" or "Eddie" for "Edward." Athletes and other famous people have many monikers. There was a very large football player for the Chicago Bears (William Perry) whose moniker was "The Refrigerator." That's a lot nicer moniker than "Ivan the Terrible."