For hundreds of years, learned people of the Western world were usually taught Latin. Contemporary scholars, however, are not often so (un?)lucky. For readers and writers of footnotes and bibliographies, this can lead to no little confusion, especially if you’re a historian dealing with sources written in previous centuries. Here’s a cheat sheet to help you out with a few commonly found but hard-to-translate terms.
et al.: This means et alia, or “and others” (CMoS, 16th ed., 10.43) In footnotes, this is used in the place of the names of multiple authors. List the first, then add et al. In the bibliography, list everyone up to ten authors. If the source has more than ten, list the first seven, then add et al. (14.76). Note that “al.” is an abbreviation, so it requires that period. “Et” is a whole word, so it does not.
ibid.: This is the abbreviation of ibidem, meaning “in the same place” (10.43). It’s used when you’re too lazy to write out all the information you used in the previous footnote. It replaces as much of the previous information as is identical, but for clarity’s sake shouldn’t be used if the previous footnote contains more than one source (14.29).
id.: Guess what? Just to make things confusing, there’s another abbreviated Latin term, extremely similar to ibid., meaning a very similar thing, that you shouldn’t use! The abbreviation of idem, “the same” (10.43), is “id.” It replaces an author’s name in a successive footnote. Unless you’re writing legal copy, avoid this . . . but now at least you know what it means (14.30)!
loc. cit.: This is an abbreviation of loco citato, “in the place cited” (14.43). It’s used to replace a work’s title, but not its author, in a footnote (e.g., Rothschadl, loc. cit., 942). Unlike ibid., it could refer to any old preceding footnote at all (not just the directly preceding footnote). It’s out of date and confusing, and CMoS argues that it should be avoided (14.31).
op. cit.: This, clearly, is loc. cit’s cousin, and is the shortened form of opere citato, or “in the work cited” (14.43). It’s used exactly like loc. cit., and it should be avoided for the same reasons.
And finally, a note applying to all of the above: Since these are words in English (though borrowed directly from Latin), no need to italicize any of them (7.52, 7.53). When in doubt on this score, check Merriam-Webster’s.