Planetoid (meaning planet-like) is an old synonym of asteroid.
The term is again coming into use, but now meaning only the largest asteroids and trans-Neptunian objects like 1 Ceres, 50000 Quaoar, 90482 Orcus or 90377 Sedna. Isaac Asimov suggested the term mesoplanet be used for planetary objects intermediate in size between Mercury and Ceres (such as Pluto and some planetoids).
In science fiction, planetoids can have the meaning of artificial planet or large, planet-like starship (e. g., Death Star)
See also