idem is a Latin term meaning "the same". It is commonly abbreviated as id., which is particularly used in legal citations to denote the previously cited source (compare ibid.). It is also used in academic citations to replace the name of a repeated author.

Id. is employed extensively in Canadian legislation and in legal documents of the United States to apply a short description to a section with the same focus as the previous.

Id. is masculine and neuter; ead. (feminine), is the abbreviation for eadem, which also translates to "the same".

As an abbreviation, Id. always takes a period (or full stop) in both British and American usage (see usage of the full stop in abbreviations). Its first known use dates back to the 14th century.