Asteroids are a class of small Solar System bodies
Small Solar System body
A small Solar System body is an object in the Solar System that is neither a planet nor a dwarf planet, nor a satellite of a planet or dwarf planet:...

 in orbit around the Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

. They have also been called planetoids, especially the larger ones. These terms have historically been applied to any astronomical object orbiting the Sun that did not show the disk of a planet and was not observed to have the characteristics of an active comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

, but as small objects in the outer Solar System were discovered, their volatile
In planetary science, volatiles are that group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane, all compounds of C, H, O...

-based surfaces were found to more closely resemble comets, and so were often distinguished from traditional asteroids.