A red dwarf is an old, cool star. Although a typical red dwarf is small by star standards, it's still bigger than Jupiter, the largest planet in our solar system.
Red dwarfs are the most common kinds of stars in the Milky Way, but they are so relatively dim and small that they can't be seen without a telescope. The star that's closest to our sun is a red dwarf called Proxima Centauri. In astronomy, cooler stars are described as red, and hotter stars are considered blue. The term red dwarf was first used around 1915.