The EELV Secondary Payload Adapter (ESPA) is an adapter for launching secondary payloads on orbital launch vehicles.
Originally developed for US launch vehicles in the 2000s to launch secondary payloads on space missions of the United States Department of Defense that used the Atlas V and Delta IV, the adapter design has become a de facto standard and is now also used for spaceflight missions on non-governmental private spacecraft missions as well. For example, multiple ESPA rings were used on a non-DoD launch of the SpaceX Falcon 9 that carried the Orbcomm OG-2 constellation of communication satellites.
The use of ESPA ring technology reduces launch costs for the primary mission and enables secondary and even tertiary missions with minimal impact to the original mission.