Atlec was an inhabited planet located in the Omega Sagitta system. In the early 2160s, this planet, and its twin Straleb, were colonized by a humanoid species. These twin planets formed the Coalition of Madena, which was created by a treaty.
A conflict between Atlec and Straleb nearly broke out in 2365 when the Straleb Jewel of Thesia was thought to have been stolen by Thadiun Okona, who was also accused of impregnating Yanar, the daughter of the leader of Atlec, Debin.
Following a confrontation involving the Federation starship USS Enterprise-D, an Atlec vessel and a Straleb security vessel, it was revealed that the jewel, which was in Okona's possession, was destined for Yanar, from Benzan, the heir of the jewel. (TNG: "The Outrageous Okona")
This planet was only mentioned in dialogue.
According to Star Trek: Star Charts ("United Federation of Planets I") and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), the Omega Sagitta system was located in the Alpha Quadrant.
The planet's name was spelled as "Altec" in the Star Trek Encyclopedia (3rd ed., p. 9) and in the Star Trek: Star Charts ("United Federation of Planets I"); however, it was spelled as "Atlec" in the script  and at StarTrek.com.