An elliptical path followed by a spacecraft moving from one orbit to another, designed to save fuel although at the expense of a longer journey time.
Space probes travel to the planets on transfer orbits. A probe aimed at Venus has to be ‘slowed down’ relative to the Earth, so that it enters an elliptical transfer orbit with its perigee (point of closest approach to the Sun) at the same distance as the orbit of Venus; towards Mars, the vehicle has to be ‘speeded up’ relative to the Earth, so that it reaches its apogee (furthest point from the Sun) at the same distance as the orbit of Mars.
Geostationary transfer orbit is the highly elliptical path followed by satellites to be placed in geostationary orbit around the Earth (an orbit coincident with Earth's rotation). A small rocket is fired at the transfer orbit's apogee to place the satellite in geostationary orbit.