(this isn't on wikipedia, and isn't easily googlable, but the concept is mentioned in some LW comments)
Similar to computronium, but instead of the matter being optimized for maximum computing power, it is optimized for maximum utility. For a paperclip maximizer, Utilitronium is paperclips. For a Classic Utilitarian, one of the most likely forms of Utilitronium is computronium that is being used to simulate minds experiencing the highest form of pleasure possible for a mind to experience.