The ArcGIS API for Flex is/was a web mapping API for Flex developers.
The ArcGIS API for Flex is/was one of several Web Mapping APIs offered by Esri. It was first introduced in 2008 as part of the ArcGIS 9.3 family. As of November 2014 development has been discontinued. Technical support by Esri will be available through June of 2016.
Resource Center: https://developers.arcgis.com/en/flex/