From Diablo Wiki
Uber is a slang term in gaming communication. It means super or supreme or ultra, and is often applied to a powered-up version of a character, item, quest, monster, etc.
The word itself is German, and literally means "above," "over," or "across." It is often used as a prefix in German, in words such as "overpass" to simply mean a road that passes over another road, without any of the "super" connotations it has in English. See the wikipedia article for more linguistic and etymological details.
 Ubers in the Diablo Games
In Diablo 2 and Diablo 3, "uber bosses" or "Ubers" are souped up versions of regular bosses with new abilities and changed appearances.
Diablo 2 featured Uber Diablo, Uber Baal, and Uber Mephisto in the so-alled Uber Tristram as part of the end game special event.). (To be specific, it's actually Pandemonium Diablo in Uber Tristram, as the name "Uber Diablo" had previously been used for another "uber" version of the demon in another
Diablo 3 got its own Ubers as part of the Infernal Machine event, where players must battle two Ubers at a time in three different levels to obtain the Demonic Organs required to create the Hellfire Ring.
- The Realm of Discord. Battle Uber versions of King Leoric and Maghda for a chance to obtain the Writhing Spine.
- The Realm of Chaos. Battle Uber versions of Ghom and Rakanoth for a chance to obtain the Devil's Fang.
- The Realm of Turmoil. Battle Uber versions of Siege Breaker and Zoltun Kulle for a chance to obtain the Vengeful Eye.