Tzenketh, also known as the Tzenkethi homeworld, was an inhabited planet in the Tzenketh system of the Alpha Quadrant. This planet was the homeworld of the Tzenkethi, a warp-capable species. (DS9: "The Adversary")
Elim Garak once conducted a mission on Tzenketh while an agent in the Obsidian Order. During the operation he was trapped when a room's walls collapsed on him, causing an attack of claustrophobia. (DS9: "By Inferno's Light")
This planet was only mentioned in dialogue. It was first mentioned in "By Inferno's Light". In "The Adversary", it was referred to as the Tzenkethi homeworld.
According to Star Trek: Star Charts (p. 36) and Stellar Cartography: The Starfleet Reference Library ("Stellar Cartography", p. 11), it was suggested through inference, with the Tzenketh system being identified as the capital system, that Tzenketh was the capital planet of the Tzenkethi Coalition (β). According to the Stellar Cartography: The Starfleet Reference Library ("The Dominion War: Strategy and Battles, 2373-75"), the Tzenkethi Coalition was neutral in the Dominion War.
In the MMO Star Trek Online, Tzenketh is present in the Alpha Quadrant, but not currently accessible. Season 12: Reckoning introduces its dominant species, the Tzenkethi, as a new adversary.