In fact, actual astronomers refer to all the solid objects that orbit the Sun as "planets". The come in three sizes: major planet, dwarf planet, and minor planet. The IAU Minor Planet Center ( http://www.minorplanetcenter.n... ) tracks all those things otherwise known as "asteroids".
The exact dividing lines are:
Major Planet - Round, and massive enough to have "cleared" it's orbit of other large object (it's the dominant mass in it's orbital region)
Dwarf Planet - Round, but has not cleared it's orbit, thus Ceres and Pluto fall into this category.
Minor Planet - Too small to become round under it's own gravity.
As a note, the stuff that got "cleared" falls into three groups: impacted one of the other planets and got absorbed, kicked entirely out of the Solar System, or kicked into an eccentric orbit but not ejected. That last group is called the "Scattered Disk", and there are around 400 known objects in the category. They are separate from the Kuiper Belt, which is leftovers in the outer Solar System which have not really been moved in their orbits. There are about 1200 objects in the Kuiper Belt, inlcuing Pluto.