1. Richard Posner, Law, Pragmatism, and Democracy 28-29 (2003). 2. Ibid. at 31.
in scholarly writing: a reference in a footnote, bibliography, etc., indicating that the citation is identical to the previous citation; though it may be followed by a qualifier such as a different page number
"Id." is often used instead of "ibid." for the same purpose in legal writing. "Op. Cit." and "Loc. cit" are close variations.
An unqualified "Ibid" or "Id" is like seeing ditto marks — telling the reader that the current citation is the same as the prior citation. The "Ibid" or "Id" may be followed with a qualifier such as a chapter or page to indicate the same place with that variation.
The first letter may or may not be capitalized. Sometimes the period is omitted.
"Ibid" is an abbreviation for the Latin word, "ibidem" (in the same place). "Id" is an abbreviation for the Latin word, "idem" (the same). Some use "Id" to reference a different book or article by the same author.