Klingon system

The Klingon system was the star system where Qo'noS, the capital of the Klingon Empire, was located. The system was located less than ninety light years from the Sol system, and an NX-class starship could travel between them in four days at warp 4.5. 

Background information
The name "Klingon system" has never been used in canon. It comes from a cut scene from, in which it was referenced by Kurn in dialogue.

The star of the system was first seen in as the USS Enterprise (NCC-1701-D) and  approached Qo'noS. According to the Starfleet Medical Reference Manual, the Klingon homeworld was the planet "Epsilon Sagittarii B". Epsilon Sagittarii is a real star located 143 light years away from Earth. According to the Star Trek: Star Charts (page 56), the star was an located in the Beta Quadrant. Star Trek Maps and The Worlds of the Federation name the star as "Klingon".

Apocrypha
The star is called "K'lai Klinzhai" in FASA's The Klingons and "K'thar" in Decipher's Worlds.

Star Trek Online refers to this system as the Qo'noS system, and establishes that Praxis is one of Qo'noS' moons.