Why is Pluto no longer a planet?

Why is Pluto no longer a planet?  or Why is Pluto called a dwarf planet?

International Astronomical Union (IAU) defined the term 'planet' in 2006. According to IAU, planets are celestial objects large enough to be made round by their gravitational orbit around the sun. A planet should be able to clear its neighbouring planetary objects and derbis.

Pluto is no longer considered a planet, and is classified as a dwarf planet because, while it is large enough to become spherical, it is not big enough to exert its orbital dominance and clear the neighbourhood of derbis materials surrounding its orbit.

IAU's definition caused the reclassification of Pluto as a dwarf planet. 

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