A lighthouse is a type of building, typically a tower, erected to emit light as a navigational aid to sea-going vessels.
In the 2250s Starfleet constructed Project Pharos, an interstellar lighthouse. The Pharos tower extended out of the atmosphere of the planet it was built on; Starfleet intended it to broadcast navigational data throughout the Marrat Nebula, and to log illegal traffic.
When the Starfleet engineers were drilling the foundations for Pharos they discovered vast quantities of dilithium - when the Klingon Kaaj learned of this he saw Starfleet's efforts an attempt to claim the open system and attacked the lighthouse. A battle between the IKS Varchas and USS Enterprise followed, and at a stalemate Captain Christopher Pike fired on the planet itself; destroying the contested planet and Pharos, but in the process starting a chain reaction in the dilithium that made the planet one huge beacon. (EV comic: "The Fires of Pharos")
Pharos is the etymological ancestor of the word lighthouse in many languages, originating from the island of Pharos in Alexandria, Egypt, the location of the ancient Lighthouse of Alexandria, one of the seven wonders of the world.