Ibid. and op. cit.

Ibid. and op. cit.

Footnotes and/or reference lists may contain the abbreviations Ibid. and/or op. cit.

DEFINITIONS
- Ibid. (abbreviation for the Latin Ibidem, meaning "The same").
  Refers to the same author and source (e.g., book, journal) in the immediately preceding reference.
- op. cit. (abbreviation for the Latin opus citatum, meaning "the work cited").
  Refers to the reference listed earlier by the same author.
Ibid. refers to the immediately preceding reference; op. cit. refers to the prior reference by the same author.
EXAMPLES
1. R. Poirer, "Learning physics," (Academic, New York, 1993), p. 4.
2. Ibid., p. 9.
3. T. Eliot, "Astrophysics," (Springer, Berlin, 1989), p. 141.
4. R. Builder, J Phys Chem 20(3) 1654-57, 1991.
5. Eliot, op. cit., p.148.
IDENTIFYING THE SOURCE
- Where Ibid. appears,
  the source is listed in the immediately preceeding reference.
  For reference # 5 in the list above, the source is listed in # 4 (Poirer, "Learning Physics").
- Where op. cit. appears,
  the source is listed in the previous reference by the same author.
  For reference # 8 in the list above, the author is Eliot and reference # 6 is by Eliot so the source is "Astrophysics".