Two References to the Same Source
What do you do when you quote more than once from the same source? Do you have to do the reference in full every time? It depends on what system of referencing you're using.
If you're using Harvard references it's no problem to put (Smith 1998 p. 23) in the text, followed three lines later by (Smith 1998 p. 49) and so on. There's only one item to put in the List of References at the end, no matter how many times you've referred to it.
If you're using numerical references, then your first footnote or endnote has all the information in it. After that, it makes sense to use a shortened form of the footnote. So John Smith, The Analysis of Complex Systems, (London: Routledge, 1998) p. 23 can become Smith, Analysis p. 49 in the second note, or even Smith (1998) p. 49.
Question: What are ibid. and op.cit.?
These are ways of handling references to the same source. (ibid. p.49) as a citation in the text or a footnote means that this reference is to the same source as the one immediately before it. (Smith op.cit. p. 49) means that you've referred to the Smith book before, but there are references to other things between the first reference to Smith and this one.