No'Mat was a location, known for its lava caves, in the mountains of the planet Qo'noS. These caves played a role in the Rite of MajQa wherein a young Klingon while meditating would experience a vision. This vision, or hallucination depending upon one's interpretation, might play a role in the Klingon's future. (TNG: "Birthright, Part I", "Rightful Heir"; DS9: "The Sword of Kahless")
When Worf was young, he ran away to the Caves of No'Mat and had a vision of Kahless, and was told that he'd do something no other Klingon had ever done. Years later, when "Kahless" returned on Boreth, Worf was convinced that he was the real Kahless when he narrated his appearance and words he had said to the young Worf. (TNG: "Rightful Heir")
According to Star Trek: Star Charts (pp. 61, 62; "United Federation of Planets IV") and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385”), there was a star system named No'Mat (Omicron Leonis), in which this planet might possibly have been located, in the Beta Quadrant. This system was located in Klingon space, in the Beta Quadrant.
The Star Trek Encyclopedia (4th ed., vol. 2, p. 84) also stated that No'Mat was a planet.
According to the script for "Birthright, Part I", this location's name was pronounced as "no-MAT".