A brown dwarf was a stellar object that had characteristics of both a star and a planet.
The Enterprise NX-01 wanted to investigate a brown star system in 2152 when the ship was infested by a symbiotic lifeform and the investigation was interrupted. (ENT: "Vox Sola")
According to Data in 2365, the abundance of dysprosium in a brown dwarf star was different than regular stars, when factored with terbium, holmium, erbium, and europium. (TNG: "Manhunt")