(Latin, short for "ibidem", "the same place")
Ibid. is the abbreviation, ibidem the full Latin word meaning “in the same place.” The abbreviated form in particular is a convention of scholarly footnoting: when you wish to cite a work and a page for which you have just given a reference, ibid. does the job succinctly.

Usage
1. R. Millan, "Latin for dummies" (Academic, New York, 1997), p. 23.
2. Ibid., p. 100.
The reference in no. 2 is the same as in 1 (R. Millan, "Latin for dummies"). Note that the page number is different.