A red dwarf was a small, cool, very faint, main sequence dwarf star.
The homeworld of Makull's species, an M-class planet, was located in a red dwarf system. USS Voyager discovered this system in the Delta Quadrant in 2371. (VOY: "Time and Again")
In 2401, red dwarf was listed as one of the results in Raffaela Musiker's search for the "red lady". (PIC: "The Next Generation")