vestigial organ
Biological term for an organ that has lost most or all of its ancestral function through evolution.
The human appendix is an example of a vestigial organ.
Synonym also meaning 'vestigial organ'; 痕跡器官 is slightly more formal and less common than 退化器官.
Compound of 退化 (たいか, 'degeneration') and 器官 (きかん, 'organ').