Xyrillian homeworld
The Xyrillian homeworld was an inhabited planet located somewhere in the Alpha or Beta Quadrant. This was the homeworld of the spacefaring Xyrillians and Thera was one of its cities.
In 2151, Xyrillian engineer Ah'len created a holographic projection of her homeworld for Trip Tucker. The simulation included Thera. The simulation also depicted pterodactyl-like life forms, presumably native to the planet. (ENT: "Unexpected")
In the episode, Ah'len calls her home "Thera". It is not explained on-screen if this was the name of the planet, or just the name of the city seen in the hologram. However, the script for "Unexpected" specified it was the name of the city.
The informal designation for this planet was from the Star Trek Encyclopedia, 4th ed., vol. 2, p. 389.