Alternate Traveller Universe
An ATU or Alternate Traveller Universe is a fan-created Traveller background or a fan-modified background using existing setting elements.
Please see the following AAB articles for more information:
One of the joys of having a tool kit like Traveller is that it is endlessly modifiable, which is exactly what fans do.
History & Background (Dossier)
There are hundreds, perhaps thousands if Alternate Traveller Universes.