Dwarf planets were celestial bodies similar to actual planets, which were nevertheless not considered to fall under the definition of a planet. They were generally smaller than planets, but larger than planetoids and asteroids.
While Pluto was considered a planet in the 20th century, it was classified as a dwarf planet Steven Taylor's times. (AUDIO: The Anachronauts) However, some still called it a planet in later eras. (TV: The Sun Makers)
Behind the scenes
In the real world, Sedna is also considered to be a dwarf planet, but has not been referred to as such in-universe.