Fire dimension

Fire Space was a pocket dimension that Megatron created using the power of Unicron. It is essentially a portion of his internal universe, but bathed in fire and resembling the inside of a volcano.

(The term is heavily used in the Japanese show Galaxy Force. It also exists in Cybertron in identical fashion.)

Cybertron

Optimus Prime pursued Megatron and fought him to a standstill. Optimus was able to win back the Omega Lock and escape Fire Space. As Megatron began to emerge in pursuit, the Omega Lock and Vector Prime's sword were used to collapse Fire Space around him, resulting in Megatron's first demise in this series.

Although Fire Space collapsed, it would be created again by Megatron on Gigantion after his defeat at the hands of Metroplex.

A portion of Fire Space was then controlled and accessed by Starscream, after losing the battle with Galvatron.

From Teletraan I: The Transformers Wiki, a Wikia wiki.