A Vestigial organ is an organ which had performed a function in our ancestor's body and is not functioning in our presently existing body.
Eg:- Appendix (found near the ceacum of large intestine), Tail bone (last bone of the backbone
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This is an organ that served a purpose in the distant ancestor of an organism, but is not longer pertinent in the presently existing, recently evolved, organisms. 
An example would be the muscles we have that can move our ears a bit. Since human ears cannot be reorientated like some animals can (and presumably our long gone ancestors from million of years ago) then those muscle do not serve much purpose (except for mild entertainment). 
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