Retrocession is the return of something (e.g., land) that was ceded in general or, specifically:
Examples:

• District of Columbia retrocession, the retrocession to Virginia, and potentially to Maryland, of the land ceded to create the District of Columbia
• Retrocession of Louisiana (New Spain) from Spain to France, formally accomplished just three weeks before the U.S. received the Louisiana Purchase lands from France
• Taiwan retrocession, the retrocession of Taiwan from Japan to China in 1945; See Taiwan under Japanese rule
  • Retrocession Day, an annual observance and unofficial holiday in Taiwan