The Xindi Empire was a potential future Xindi government which the Xindi-Reptilians might have someday led. It was referred to from the perspective of the 22nd century. (ENT: "The Council", "Zero Hour")

The notion of a Xindi empire that would be truly united, and stronger than the Xindi's 22nd-century council-led alliance, was referred to the Xindi by the Sphere-Builders (who were otherwise known to the Xindi as "the Guardians"). The concept was referenced by a female Sphere-Builder who was trying to persuade the Reptilians to betray most of the other Xindi species by launching the Xindi weapon against Earth without the authority of the Xindi Council; the purportedly foreseeable empire was said to eventually result from the Reptilians and Xindi-Insectoids nonetheless deploying the weapon.

Furthermore, the same Sphere-Builder told Reptilian Commander Dolim, since she was holographically appearing on the bridge of his Xindi-Reptilian warship, that the Sphere-Builders would guarantee Reptilian rulership of the Xindi Empire if Humanity was eliminated. (ENT: "The Council") Fully convinced by the Sphere-Builder, Dolim later relayed the idea to some of the officers under his command, while they were alone together. (ENT: "Zero Hour")

Given the way the Sphere-Builders used the notion of a Xindi empire to manipulate Dolim, it is unclear if they really foresaw such a future, or just made it up.
In the script of "The Council", the name of this hypothetical government was formatted as "Xindi empire". However, it was formatted as "Xindi Empire" in the script of "Zero Hour".
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