Multiverse

The multiverse, as revealed in "The Lich," is comprised of an infinite amount of conceivable dimensions. Dimensions can become linked to each other temporarily by naturally-occurring wormholes or artificially via powerful artifacts, such as the Enchiridion.

Map of Multiverse
When the sword on the Enchiridion is turned sideways it displays a holographic map of the known dimensions of the multiverse as well as the connections between them. Dimensions displayed on the map come in a wide variety of three dimensional shapes. Whether these shapes are true reflections of the dimensions or are symbols to help distnguish individual dimensions is unknown.

"Home" Dimension
This is where all episodes of the series had taken place prior to the events of "The Lich". In the map of the multiverse it's portrayed as a sphere surrounded by a spinning cube.

The Time Room
All information about the Time Room comes from Booko. It is said to lay at the center of the multiverse and is the only dimension to exist outside of time. Time waves produced from here are felt by other dimensions. According to Booko, the Time Room is quasi-corporeal and is believed to be the dwelling place of "the almighty Prismo" On the map of the multiverse, it is portrayed as a cube with smaller cuboid shaped holes cut out of the center of each side, possibly from one side of the cube to the other.

The Enchiridion
When all nine jewels are placed in the Enchiridion, it gains the ability to open a portal to any dimension in the multiverse. When Finn placed the last jewel into the Enchiridion, it turned into a stone tablet. When Finn shattered the tablet to prevent the Lich from obtaining it, he only succeeded in opening the portal. While the Enchiridion itself appears to have been destroyed, the jewels placed inside it were left behind when the portal closed.