Thought for sure it was "Ibid". An abbreviation, perhaps?
Both "Id." and "Ibid." are abbreviations of "idem" and "ibidem" respectively meaning "the same" and "in the same" respectively. There used to be a distinction made between the use of the two terms and their respective abbreviations, but the exact distinction differed and now almost no one really cares but as a general rule legal citations use "Id."
(When a distinction is made, ibidem is always a more exact match of the previous citation than idem. Some hold that ibidem should only be used if the exact same citation, without any changes at all and thus to cite another part of the same work, "Id. at *89" has to used. Others hold "idem" should be used only if the author is the same but a different work is being cited, thus to cite another part of the same work "Ibid. at *89" is required. Consult the style guide used by whomever you submit your work.)