Technically, there was never a scientific definition of the term Planet before 2006. When the Greeks observed the sky thousands of years ago, they discovered objects that acted differently than stars. These points of light seemed to wander around the sky throughout the year. We get the term "planet" from the Greek word "Planetes" - meaning wanderer.
Astronomers of the International Astronomical Union (IAU) voted on and passed the first scientific definition of a planet in August 2006. According to this new definition, an object must meet three criteria in order to be classified as a planet. First, it must orbit the Sun. Second, it must be big enough for gravity to squash it into a round ball.
And third, it must have cleared other objects out of the way in its orbital neighborhood. To clear an orbit, a planet must be big enough to pull neighboring objects into the planet itself or sling-shot them around the planet and shoot them off into outer space. According to the IAU, Pluto does not meet this third requirement but is now in a new class of objects called "dwarf planets." It is this third part of the definition that has sparked debate.