What is the closest satellite to the Earth?

What do we mean when we ask about the closest satellite to the Earth? Is it referring to a man-made object orbiting our planet or a natural satellite, like a moon? And how do we determine what qualifies as "closest"? Is it based on the distance from the Earth's surface or from its center? Additionally, are there any factors that could affect the position of the closest satellite, such as gravitational pulls from other celestial bodies or changes in the satellite's orbit? Understanding these details will help us identify the answer to the question, "What is the closest satellite to the Earth?