In legal or academic writing, idem is used in footnotes to mean "the same as previously mentioned." For a research paper, you can use idem to indicate you're citing a different page of the exact same source.
The Latin idem means "the same," and it's usually abbreviated as id. These days, most academics prefer to use the term ibid., or "in the same place," in scholarly bibliographies, or to simply repeat the author's last name. If you do come across idem in footnotes, endnotes, or a legal document, you'll know it means "this information comes from the same work and author as the last citation."