The Equator is an imaginary line that runs around the Earth, midway between the North and South poles. It divides the Earth into the Northern Hemisphere and the Southern Hemisphere. Option 1, a line joining the North and South poles, is incorrect because the Equator does not connect the poles directly. Option 3, a belt around the planet Saturn, is also incorrect as the Equator is specific to Earth and does not apply to other celestial bodies. Option 4, the axis of rotation of Earth, is incorrect as the Equator is perpendicular to the axis of rotation. Therefore, the correct answer is option 2, an imaginary line passing around the Earth, dividing it into the Northern and Southern Hemisphere.