Wiktionary
n. Any object (usually a minor planet or a hypothetical planet) in the Solar System with all or most of its orbit outside the orbit of Neptune
Wikipedia
A trans-Neptunian object (TNO, also written transneptunian object) is any minor planet in the Solar System that orbits the Sun at a greater average distance ( semi-major axis) than Neptune, 30 astronomical units (AU). Twelve minor planets with a semi-major axis greater than 150 AU and perihelion greater than 30 AU are known, which are called extreme trans-Neptunian objects (ETNOs).
The first trans-Neptunian object to be discovered was in 1930. It took until 1992 to discover a second trans-Neptunian object orbiting the Sun directly, . over 1,750 trans-Neptunian objects appear on the Minor Planet Center's List Of Transneptunian Objects. Of these TNOs, 1,563 have a perihelion farther out than Neptune (30.1 AU). , two hundred of these have their orbits well-enough determined that they have been given a permanent minor planet designation.
The largest known trans-Neptunian object is Pluto, followed by , , and . The Kuiper belt, scattered disk, and Oort cloud are three conventional divisions of this volume of space, though treatments vary and a few objects such as do not fit easily into any division.