Ibid is a contraction of ibidem, a Latin word meaning “the same place.” This term is most commonly used for footnoting in scholarly texts, allowing the author to use it instead of citing a lengthy title. In legal texts, people may use “id,” a shortening of “idem,” a word that means “as mentioned previously.”
Essentially, this term is a fancy form of ditto marks. If, for example, a writer is referencing something like The Effects of Factory-Produced Emissions on the Greater Nile Watershed: An Environmental Study, that's a long title to have to refer to again and again. Instead, he or she can reference the title in a footnote, and then use “ibid” in future footnotes. If the writer moves to a new location in the text, he or she can alert your readers with “Ibid (page 23)” or a similar note, depending on what kind of citation format you are using.
When a new source is introduced, the process begins all over again. In other words, if the writer cites The Effects of Factory-Produced Emissions on the Greater Nile Watershed: An Environmental Study once and follows with four additional citations marked with “ibid” before moving on to Cultural Practices in the Southern Nile Floodplain, when the term is used after this source, it would refer to Cultural Practices in the Southern Nile Floodplain, not to the original text.
The use of “ibid” refers to both an author and a text. In some scholarly texts, people use “idem” to refer to an author alone if multiple works by the same author are being cited, as in “John Smith, Purple Elephants After Midnight (Saint Louis, University of Missouri Press, 1974), 367; idem, Under the Lily Trees (New York, St. Martin's Press, 1981), 284.” While replacing an author's name with “idem” might seem a bit lazy, some authors do have rather lengthy names and titles, and the term saves space and time.
Different scholarly disciplines have different rules about citations, and specific instructors, universities, and publications may have their own preference. For this reason, it is a good idea for a writer to consult a style manual before submitting material for publication, grading, or evaluation, to ensure that it meets the basic stylistic standards for the venue to which is it being submitted. While making small adjustments to footnoting practices may seem nitpicky to some, it ensures that material is standardized, and submissions that have not been properly edited may be rejected.