Answer: Osteochondritis dissecans or Osteochondral Defect

Large loose, but not displaced, bony fragment in the classic lateral aspect of the medial femoral condyle. Osteochondritis is the name for the avascular necrosis causing the fragmentation with osteochondral lesion (or defect) the name for the end stage loose body and bony defect.

And yes, there are stages! https://radiopaedia.org/articles/osteochondral-injury-staging