The term "idem" is used when referring to something that has already been mentioned. It often refers to an author, an article or a chapter that was previously cited. It can be used as a pronoun or an adjective; idem is a Latin word that originated between 1350 and 1400.
Idem or its abbreviation "Id.", is used in legal and academic citations. For example, when referring to an author previously mentioned, "Id." is used instead of the name of that author. "Id." is always followed by a period in both American and British usage because the final two letters are not present.