Limbo, as it is referred to in the dream realm, is an expanse of infinite raw subconscious.
Limbo exists as a space that is not dreamt by any one individual, but is a shared space where any mind can make drastic alterations of any kind.
It is to be understood that death within a heavily sedated sleep will send the sleeper's mind to Limbo, but as shown by Fischer in the Mountain dream, when one dies and goes to Limbo, the sleeper's 'dead' body actually behaves more like a sleeping body. Eames is able to administer a kick to Fischer in the form of the defibrillator, that, when synchronized with a kick from within Limbo, can wake him up back in the Mountain Dream where he "died".
Traveling to LimboEdit
Limbo can be entered by traveling deeper through the dream levels. After Fischer dies in the Mountain Dream, Cobb and Ariadne are able to enter Limbo by sleeping. The only reason this is possible is because, as Yusuf establishes earlier, the team cannot create more than three stable levels of dreams. Therefore, sleeping in Level 3 results in a sleeper dreaming themselves into Limbo. It should also be noted that manually entering Limbo allows a sleeper to retain the knowledge that they are dreaming, making it easier to avoid becoming 'lost' in Limbo.
Dom and Mal's ExperimentsEdit
At some unspecified point in time before the Fischer Inception Job began, Cobb and Mal were performing experiments to see how deep they could travel within their minds. Eventually they manually dreamt themselves into Limbo. Here they were able to create a perfect environment together and feel like gods. Eventually, the couple began to build from their memories, making the distinction between reality and dream more difficult to distinguish. Over time, Cobb became dissatisfied with Limbo because he knew that it was not real, while Mal was beginning to accept it as her reality. Mal would not accept that she was asleep so Cobb was forced to perform Inception on her, planting the idea in her mind that her world was not real. At this point Mal was filled with enough doubt to allow her and Cobb to kill themselves on train tracks in order to wake up. After waking, the idea Cobb planted in her mind was still growing and Mal still believed she was dreaming. Eventually, she committed suicide in an attempt to wake up. The guilt of knowing that his Inception caused her to kill herself continues to haunt Cobb.
The time scale in limbo is stretched out to a much higher extent than in dreams since it is not a dream level, but a separate plane. Very few people have experimented and gone into limbo due to its risks. As a result of the extreme time dilation in the plane, people in limbo can experience years in a minimal amount of time. The resulting effect on the mental state can be difficult to overcome upon waking, as the years experienced in limbo can result in the dreamer believing that what they have experienced is actual reality.