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")
The Star Trek Encyclopedia (3rd ed., p. 325) 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".