The term Asteroid is used for Minor Planets in the Asteroid Belt, out to Jupiter's orbit. It is sometimes used more generally: it is often used to include minor planets closer to the Sun, such as those that are Near-Earth Objects, and sometimes used to include Centaurs, i.e., minor planets further out.
The first discovered asteroids were Ceres and Pallas in 1801, with the 15th discovered in 1851, and were termed asteroids which means star-like since they appeared as points (but moved like planets). Now 100,000 have been identified, many by computer, and the estimated population is millions.
A diameter of 10 meters is often (but not always) used as the threshold between asteroid and Meteoroid. Ceres, the largest in the asteroid belt, is near 1,000 km in diameter, the next largest with less than 600 km. Asteroids are generally classified by type of orbit and by spectrum.
Binary Minor Planet (BMP)
Corotation Resonance (CR)
Catalina Sky Survey (CSS)
Kilometer Size Barrier
Kuiper Belt (K Belt)
Late Heavy Bombardment (LHB)
Minor Planet Center (MPC)
Near-Earth Object (NEO)
Nearby Supernova Factory (NSNF)
Palomar Transient Factory (PTF)
Wide-Field Infrared Survey Explorer (WISE)