Apud is a latin word that means with, Close of, in. It is used in bibliographic references to make an indirect citation, that is, cite an excerpt that was not read directly in the original work, but cited by another author.
It is usually used in the body of the text with the meaning of "quoted by", "second" or "as per", indicating the reader that the citation is made according to what was read and referenced by another author who had access to the work original.
The term very often appears in theses, articles, monographs and other academic dissertations. However, it is advisable throughout the work not to quote too many using the word apud, and it should only be used in cases of hard-to-access original works, for example: old publications, rare works or texts in languages that are difficult to access.
An example of the use of the term is: Euclides, apud Boyer, 1991.
If in a work the reference above appears, through the term apud it is known that the author of the work did not have access to the work written by Euclides. However, he had access to the work written by Boyer where he quoted from the original work.
The rules for citing authors are regulated by the ABNT (Brazilian Association of Technical Standards).