From Halopedia, the Halo wiki
A thema was a semi-physical and logical division of the galaxy created by the Forerunners to segment their ecumene. Each thema contained a number of star systems. A thema was bordered by massive rippling magnetic fields of a galactic scale.
Forerunner displays simplified and projected the complex variability of a thema boundary as a curtain of colored light. These borders were detectable in normal space and slipspace. From the projected color of the field and the vortices within it, the overall health of slipspace could be determined.
A thema or margin arc infested by the Flood was known as a Burn. By the end of the Forerunner-Flood war, the boundaries of themas became more unstable as the Precursor constructs used by the Flood gradually converted slipspace to become incompatible with Forerunner slipspace drives.
List of appearances