Ibid. has been used since the 17th century as an abbreviation of the Latin word ibidem, which means "in the same place." It indicates that a cited reference is from the same source as a previous reference.
1. Warren A. Seavey, Studies in Agency (1949)
2. Ibid., Handbook of the Law of Agency (1964)
It may be used several times in succession. In footnotes or endnotes, it can be used without a page number to indicate the same page of the same source referred to in a preceding note.
10. Simone de Beauvoir, The Second Sex, trans. and ed. H. M. Parsley (1953; reprint, Knopf, 1987), 600.
11. Ibid., 609
In older works, you might encounter the similarly used Latin abbreviations op. cit. (for opere citato, "in the work cited") and loc. cit. (for loco citato, "in the place cited"). Op. cit. refers to the source cited earlier (with other notes intervening) but not necessarily to the same page or pages.
19. Sheldon and Eleanor Glueck, Unraveling Juvenile Delinquency (New York: Commonwealth Fund, 1950), 23.
20. Don C. Gibbons, Delinquent Behavior (Englewood Cliffs, N.J.: Prentice-Hall, 1972), 341.
21. Glueck, op. cit., 30-34.
Loc. cit. refers strictly to the same page or pages of the same source cited earlier, with references to other sources intervening.
13. W. T. Sanders, Cultural Ecology of the Teotihuacan Valley (State College, Penn.: Pennsylvania State Univ., 1965), 312-13.
14. Sabloff and Andrews, op. cit., 160.
15. Sanders, loc. cit.
You might also encounter the less-common abbreviation id. for idem. In Latin, idem means "same," and Old English writers borrowed it to refer to a word, phrase, etc., that is the same as something previously mentioned. Today, it is chiefly found in bibliographies to avoid repetition of an author's name and title when a reference to an item immediately follows another to the same item.
1. Whitney 50-55
3. Id., 51