Good question! According to the Nasa website, Pluto was re-classified as a dwarf planet in 2006 dwarf planet

In 2003 a group of astronomers observed a Pluto- sized body orbiting in a cold-icy zone beyond Neptune; this dwarf-planet became known as Eris. The observation of this body was confirmed in 2005. A year later, in 2006, the International Astronomical Union (IAU) met to discuss how planets should be classified profile for planets

Should Eris become the 10th planet, or should Pluto lose its planet status? They decided that Pluto should lose its planet status and that it should belong new classification of solar system bodies, known as dwarf planets.

The definition of a dwarf planet is interesting. According to the IAU, a dwarf planet is large enough to be round (a sphere) but not large enough to scatter or attract objects near their orbits. Planets and dwarf planets have strong enough gravity that they can form into a sphere. Smaller bodies, like asteroids and meteors, are irregularly shaped.