What is the origin of the term "planet" and how did astronomers first define the term?
It was defined as what the name states: πλάνητες ἀστέρες (Planetes asteres) means 'wandering stars' in Greek. By this, any light on the nocturnal firmament that didn't move along with the other stars was called a wandering star while the other stars were called 'fixed stars' since they remained always on the same location on the firmament relative to the other fixed stars, while wandering stars move on their own paths. Wandering stars (planets) included the Sun and the Moon as well. They didn't include the Earth as yet.