I have appended something to the end of something else, but I don't know the word to describe the what has been appended. Even after it is one big "something", I was wondering if there was a word that described the piece that was not always there before.
"Append" is the verb I am looking to convert. "Appension" and "appendee" are not real words as far as I know.
Edit: I have not considered "appendix" since it seems to imply being at the end specifically, rather than something that was added (anywhere) to something else.
@V0ight suggested affix, which seems by its definition to be much closer to what I had in mind!