Mesoplanet is a term coined by Isaac Asimov to refer to planetary bodies with sizes smaller than Mercury but larger than 1 Ceres. At the time Asimov coined the term, it included only Pluto; now, however, the classification would also include 90377 Sedna, 90482 Orcus, 50000 Quaoar, 28978 Ixion, and 20000 Varuna if size is defined by radius (the masses of many planetary bodies are not accurately known).

The term was coined in Asimov's essay "What's in a Name?", which first appeared in The Los Angeles Times in the late 1980s and was reprinted in his 1991 book Frontiers. In the essay, Asimov noted that the solar system has a large number of planetary bodies (as opposed to the sun and natural satellites) and stated that lines dividing "major planets" from minor planets (asteroids) were necessarily arbitrary. Asimov then pointed out that there was a large gap in size between Mercury, the smallest planetary body that was considered to be undoubtedly a major planet, and Ceres, the largest planetary body that was considered to be undoubtedly a minor planet, and that only one planetary body known at the time, Pluto, fell within the gap. Rather than arbitrarily decide whether Pluto belonged with the major planets or the minor planets, Asimov suggested that any planetary body that fell within the size gap between Mercury and Ceres be called a mesoplanet, since meso in the Greek language means "intermediate."

Since the time of the essay, several other bodies have been discovered that lie in size between Mercury and Ceres, which arguably strengthens the case for a special classification such as mesoplanet; the older term planetoid has also been revived to refer to intermediate-sized planetary bodies. On the other hand, by filling the apparent size gap between Mercury and Ceres, the discovery of such bodies seems to emphasize that any line-drawing definition of planet is an arbitrary one.