The term "planet" originates from ancient Greek, where it meant "wandering star." In ancient times, people observed celestial objects that moved against the backdrop of fixed stars. These objects included the Sun, the Moon, and the five planets visible to the naked eye: Mercury, Venus, Mars, Jupiter, and Saturn. The term "planet" was used to describe these wandering celestial bodies.
When it comes to Earth, even though we now know it is a spherical object, it was also considered a "planet" based on its motion and behavior in the sky. The ancient Greeks, as well as later civilizations, saw the Earth as one of the celestial objects moving through the heavens. The understanding of Earth's sphericity came later through scientific observations and understanding.
If the Earth were flat, it is difficult to say how the terminology might have developed. The term "planet" may not have been used to describe Earth specifically in that case. It's important to note that the concept of a flat Earth has long been disproven through scientific evidence and observation. The term "rotondeta" is not a commonly used term to describe a flat Earth; it appears to be a neologism combining the Italian word "rotondo" (round) with the diminutive suffix "-eta."