Retrogression occurs when the priority date for a particular visa category becomes unavailable or moves backward on the visa bulletin, causing delays in the processing of green card applications for individuals with affected priority dates. The priority date must be current at the time of filing the I-485 application or immigrant visa application and at the time of adjudication. Retrogression can impact an individual’s green card processing time, and the process can only move forward once the priority date becomes current.