The mindscape is the dimension inside the mind. Every sentient being has a mindscape, and every mindscape is different. It typically reflects the characteristics and memories of the person, but every mindscape has something in common - at their heart is some kind of building that houses the person's memories, thoughts, fears, and other such objects. They are usually hidden behind doors.
It is possible to enter your own mindscape while dreaming, or by using Indigo Flames to enter anyone's mindscape. While inside a mindscape, one has unlimited power due to residing in a realm of imagination. If one has a split personality, the mindscape becomes extremely chaotic, and even more so during a Shatter. After the conclusion of a Shatter or during totality, things return to relative normalcy.