A map of a populated area on Tilonus IV
Tilonus IV was the inhabited fourth planet in its star system. It was the homeworld for the Tilonians, a highly advanced humanoid species, and for the spiny lobe-fish.
A Starfleet research team was on the planet. They were housed in a small building in the southwest quarter of a city. This quarter's area was two hundred square kilometers.
In 2369, after the assassination of the Prime Minister, the government of Tilonus IV collapsed into anarchy. The remnants of the government splintered into factions that were vying for control and were desperately looking for technology or weapons that would aid them in their cause. Some factions resorted to torture for information. Fearing for their safety, the research team on the planet went into hiding.
Aboard the USS Enterprise-D, Captain Jean-Luc Picard ordered Commander William T. Riker to locate and rescue the research team there. He was captured by one of the factions and subjected to a neurosomatic technique used by Suna in an attempt at extracting strategic information from him. (TNG: "Frame of Mind")
According to the script, the pronunciation for Tilonus was "tie-LONE-us". [1]
According to the Star Trek Encyclopedia, 4th ed., vol. 2, p. 398, Tilonus IV was a class M planet.