WBU stands for the World Boxing Union. It is another organization that hands out professional boxing titles, but it is a much smaller one — outside the four that people actually count (IBF, WBA, WBC, WBO). Boxing has a lot of these smaller bodies, and each of them gives out belts, so a fighter can truthfully say 'I am a world champion' while most fans have never heard of the title. That is worth knowing: in boxing, 'world champion' is not one clear rank but a claim that depends on who gave you the belt. A WBU title is a real title and a real achievement — fighters train hard for it — but it does not carry the weight of a WBC or WBA belt, and a WBU champion is not in the conversation about who is best in the world. You will see the letters mostly in older reports from the 1990s and 2000s, and mostly around British and European fighters. Like the others, it is said letter by letter and used before a noun: the WBU title.