This article explains that "synthetic full" refers to backups synthesized by the appliance to function as a full backup.
The term synthetic full refers to backups that capture all data on a VM, but were synthesized by the appliance using existing backups. More specifically, the existing incremental backups are superimposed on the last full backup to create a new backup. Similar to full backups, synthetic full backups create a new backup group.
See also When are synthetic backups created?