Op. cit. is an abbreviation of the Latin phrase or , meaning 'the work cited' or 'in the cited work', respectively. Various different styles call for other alternatives, such as a reference to the author's surname and publication year, e.g. "Smith 1999".<!--Is this MLA style? A citation for it needs to be added from a major style guide, common as it may be.-->
Op. cit. is contrasted with ibid., an abbreviation of the Latin adverb ibidem, meaning "in the same place; in that very place" which refers the reader to the title of the work in the preceding footnote. The easily confused idem (sometimes abbreviated id.), the Latin definitive pronoun meaning "the same" is also used on occasion (especially in legal writing) within footnotes, and is a stand-in for the last-cited author, rather than title.