Footnote notation is a system of abbreviations used to refer to sources in footnotes or endnotes. The most common abbreviations are IBID (4 letters), IBIDEM (6 letters), IDEM (4 letters), ETAL (4 letters) and OPCIT (5 letters). IBID is an abbreviation for the Latin phrase 'ibidem', which means 'in the same place'. IBIDEM is an abbreviation for the Latin phrase 'ibidem, IDEM', which means 'in the same place, the same thing'. IDEM is an abbreviation for the Latin phrase 'IDEM', which means 'the same'. ETAL is an abbreviation for the Latin phrase 'et alii', which means 'and others'. OPCIT is an abbreviation for the Latin phrase 'op. cit.', which means 'in the work cited'.