supplementary material that is collected and appended at the back of a book
"The professor asked us to review the appendix in our history textbook before the final exam because it contained all the primary source documents."
a vestigial process that extends from the lower end of the cecum and that resembles a small pouch
"The surgeon confirmed that his appendix was inflamed, which is why he needed urgent surgery."
Something attached to something else; an attachment or accompaniment.
"The chef included a small appendix of handwritten notes detailing his secret spice blends at the back of the cookbook."
In plain English: The appendix is a small, tube-like pouch attached to your large intestine that usually has no important job in your body.
"The doctor said I needed surgery to remove my appendix."
Usage: Use appendix to refer to a supplementary section at the end of a book containing extra information like indexes or glossaries. Do not use it to describe the biological organ in your body when discussing general writing contexts, as that is its specific medical meaning.
The word comes from the Latin appendix, which originally meant something attached or hung on to a main text. It entered English with that same sense of an addition or supplementary section.