An abbreviation for the Latin ‘opere citato’ which means ‘in the work cited’. This abbreviation is often used in footnotes combined with an author name and, sometimes, a date. It is referring you to a source, the details of which have already been given in full or which you will find in the bibliography. Whilst ibid refers to the reference that has just been mentioned, op. cit. usually refers to a reference earlier in the text or in the bibliography, not the one that has just been mentioned. The author's name and date will help you to identify the correct source.