It refers to any part of a building that has been fabricated at a place other than its final location.  For this reason, it can be referred to by other names such as off-site fabrication, off-site construction or off-site manufacture.

Prefab is short for prefabrication.

Prefabrication is an ‘umbrella’ term and it covers a range of different systems and processes.  These will include structural, architectural and services elements.

Source: PrefabAus