The Amleth system was a planetary system located in a emission nebula, which rendered cloaking devices useless. One of its planets, Amleth Prime, was the site of a Cardassian military base. (DS9: "Return to Grace")

This system was only mentioned in dialogue.
According to Star Trek: Star Charts (p. 46) and Stellar Cartography: The Starfleet Reference Library ("Stellar Cartography", p. 18-19; "Federation Historical Highlights, 2161-2385"), the Amleth system was located in the Alpha Quadrant. This was a binary system with a M-class star and a G-class star. According to a map of the Cardassian Union – prepared by an anonymous astrography clerk, from the Cardassian Central Archives, in 2364 – Amleth was identified as a Cardassian system.