What is a transfer orbit?
A transfer orbit is an intermediate trajectory used to move a spacecraft from one orbit to another more efficiently than a direct insertion burn. The most common example is the Geostationary Transfer Orbit (GTO): because launching directly to GEO requires far more propellant than most rockets can provide, the launch vehicle places the satellite in an ellipse with its perigee at LEO altitude (~185 km) and its apogee at GEO altitude (35,786 km). The satellite then fires its own apogee engine to circularise the orbit at GEO.
Delta-v requirements
The total delta-v to reach GEO from Earth's surface is approximately 10.0 km/s. The launch vehicle provides approximately 9.2 km/s to reach GTO; the satellite's apogee engine provides the remaining ~1.8 km/s to circularise and remove inclination at GEO altitude. This apogee engine burn has historically been the largest propulsion demand on a GEO satellite's propellant budget — a key driver of satellite mass and cost.
Super-synchronous transfer orbits
Modern GEO satellite launches often use a super-synchronous GTO (SGTO), where the apogee is raised above GEO altitude (to 50,000–90,000 km). This allows the apogee engine burn to be performed at a higher altitude where the delta-v to circularise is lower, reducing propellant consumption. SGTO is now standard for direct-inject GTO launches on Ariane 6 and Falcon 9 to GEO. Electric propulsion satellites launched to GTO conduct multiple perigee and apogee burns over months to spiral up to GEO — a low-thrust transfer orbit sequence that minimises propellant at the cost of months of transit through the Van Allen belts.