What decision did the international astronomical union make in august 2006

The international astronomical union formulated a new definition of what it means to be a planet and declassified Pluto as a planet in august 2006.

Explanation: Hi, on 24 August 2006, Pluto was declassified as a planet by The International Astronomical Union following a vote at their General Assembly in Prague. It was given the status instead as a 'dwarf planet', on account of its small size - smaller than the Moon - and highly elliptical tilted orbit which overlaps with that of Neptune.

The IAU formulated a new definition of what it means to be a planet.  

A planet is a celestial body that

(a) is in orbit around the Sun,

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and

(c) has cleared the neighbourhood around its orbit.