From The Collaborative International Dictionary of English v.0.48 [gcide]:
Asteroid \As"ter*oid\, n. [Gr. ? starlike, starry; 'asth`r star
+ e'i^dos form: cf. F. ast['e]ro["i]de. See Aster.]
A starlike body; esp. one of the numerous small planets whose
orbits lie between those of Mars and Jupiter; -- called also
planetoids and minor planets.
[1913 Webster]
From WordNet (r) 3.0 (2006) [wn]:
asteroid
adj 1: shaped like a star [syn: asteroid, star-shaped]
n 1: any of numerous small celestial bodies composed of rock and
metal that move around the sun (mainly between the orbits
of Mars and Jupiter)