"Unihan variants" are alternates of characters belonging to the "Unified Han," or "Unihan," section of the Unicode standard. So-called Han characters have been used across East Asia for milennia. Over time and across regions, different forms of the same character have arisen and persisted. As recently as the 20th century, new forms of these characters have been invented and used.
Accounting for these unihan variants has been one significant challenge for the Unicode project. The Unicode project has tried to incorporate significant variants for characters, and it maintains a list of variants for each character in the Unicode standard. This list is not exhaustive, and others have compiled more exhaustive lists.