A Hohmann orbit, also known as a Hohmann transfer orbit, is an elliptical trajectory, named after Walter Hohmann, along which a spacecraft may move from one orbit to another with the minimum expenditure of energy. Such an orbit just touches the original and destination orbits, and may be used for changing the orbit of an Earth satellite or for sending a probe to another planet. It involves two firings of the spacecraft's engine: one to break out of the original orbit and another to enter the destination orbit. Its chief disadvantage is that it requires relatively long flight times. This can be overcome by judicious use of gravity-assists.
To better understand the Hohmann orbit, consider the diagram to the right. A vehicle is traveling in orbit A around the Earth, and we want to get it to orbit C. At some point, the engine performs a posigrade (fired in the direction of the spacecraft's motion) burn, thus, enlarging the orbit; the vehicle is now traveling along orbit B. The point where the posigrade burn takes place becomes the point of perigee of the new orbit (B). Unless there is a further burn, the vehicle will now continue to move in orbit B. Since we want to move the vehicle to orbit C, the size of the posigrade burn (at perigee) is well designed to ensure that the point of apogee (of orbit B) meets orbit C. At apogee, a further posigrade burn is used to enlarge the orbit again. This time, the vehicle goes into orbit C, and the transfer is complete.