The Inner Worlds referred to the inner-galactic planets of the Mutter's Spiral galaxy, as opposed to frontier worlds on the Outer Rim. (AUDIO: Bad Day in Tinseltown) Big corporations and most of the people on the Inner Worlds used their own local time, the days and years not always the same length as Earth. (PROSE: Just War)
During the Second Dalek War in the 26th century, the Auros populations were attempting to go to the Inner Worlds after their planet was destroyed. (PROSE: Prisoner of the Daleks)
When a phase shifted Cyber-Warship exploded, because its co-ordinates were set for the Inner Worlds, it left a "scar in hyperspace" that formed a wormhole from the Inner Worlds to an Outer Rim planet. (AUDIO: Bad Day in Tinseltown)