A Hohmann transfer orbit is an efficient way to move a spacecraft between two circular orbits around a celestial body, such as a planet. It involves two engine burns: the first burn increases the spacecraft's speed to reach an elliptical orbit that touches both the lower and higher orbits, while the second burn occurs at the higher orbit to circularize it. This method is named after Walter Hohmann, a German engineer who proposed it in 1925. The Hohmann transfer is commonly used in space missions because it minimizes fuel consumption, making it a cost-effective choice for traveling between different orbits.