Free US Law Dictionary

Ibid.

Ibid. (Latin, short for ibidem, "the same place") is the term used to provide an endnote or footnote citation or reference for a source that was cited in the preceding endnote or footnote. It is similar in meaning to idem (meaning something that has been mentioned previously; the same) abbreviated "Id.," which is commonly used in legal citation.[1]

To find the ibid. source, one has to look at the reference right before it, and so ibid. serves a similar purpose to ditto marks (〃, ", do.).