The term 'Pole-to-Pole Line' refers to the imaginary line that runs from the North Pole to the South Pole. This line is also known as the Earth's AXIS, which is a four-letter word that can be used to solve the crossword puzzle. The Earth's AXIS is an imaginary line that passes through the center of the Earth and is used to measure the rotation of the planet. It is also used to determine the direction of the Earth's magnetic field.