What is the definition of a planet?
Image of the Greek Parthanon under a starry night sky.
Before telescopes, scientists relied on their naked eye and careful observations to catalog the night sky. The five planets easily visible with the naked eye have been observed for all of human history: Mercury, Venus, Mars, Jupiter and Saturn.
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.