If you look throughout the history for the definition of what really a planet has actually changed always with time and meant different things at the same moment depending on what kind of person was defining it. Now we all know about Ceres because of advanced technology and telescopes. Ceres was discovered in 1801 and was in start thought to be a planet like our home Earth and other known planets of our solar system until astronomer discovered another heavy body in space named "Pallas" that has a similar orbit which affected their thought about Ceres. Astronomers of that time, even using the available technology of their time, were somehow able to tell that these objects were not actually planets. The famous astronomer of that time Sir William Herschel suggested the name "asteroids" for this kind of bodies in space and which got stuck with time.Asteroids at that time were then accepted as a totally distinct category of bodies in space.
A few years ago, you might have said that a planet can be defined as one of the nine large and heavy celestial bodies that orbit the Sun. However, with new technology, which also made the discovery of many other new celestial bodies in various regions of our solar system, such as the Kuiper Belt, also created problems for astronomers like determining "what a planet is" a little more difficult. While many of astronomers suggested various definitions for the term planets over the years but unfortunately none of them were widely accepted by other astronomers.
The issue of defining what planet is when came to a head in 2005 when an object even larger than Pluto was discovered far beyond the Kuiper Belt by astronomers. This object, which is now known as Eris, was a source of division among many astronomers. Some astronomers wanted Eris to be the part of our solar system as a tenth planet while others just considered it to be just another asteroid like many others in Kuiper belt, despite the fact that it is larger than Pluto is. The International Astronomical Union (IAU), which usually resolves problems and issues like this, met in 2005 at a conference, but instead of debating and resolving this issue, they did not come up with any agreement upon the definition of a planet. After the conference of 2005, the matter was resumed in summer of 2006 again at the next IAU conference.
In August 2006, the IAU finally agreed in conference upon a definition for “what actually a planet is”. The IAU’s official definition for defining a planet now is:-
“A planet is a celestial body that (1) is orbiting the Sun, (b) and it must have sufficient mass for its self-gravity in order to overcome rigid body forces so that it assumes a definite hydro-static equilibrium (nearly round) shape, and (c) must has cleared the neighborhood around its orbit.”
An object that has cleared the path of its orbit and neighborhood of its orbit and it is of sufficient size in order to retain its nearly round shape with the help of self-gravity. While defining “what a planet is”, the IAU also created a new category for some bodies orbiting the Sun as a “dwarf planets”, after which Pluto was categorized as a dwarf planet. Eris and several other objects were also categorized as a dwarf planet depending on their characteristics. The definition has had severe opposition since, especially with many people and astronomers angry at the demotion of Pluto (Which actually does not matter at all).