(bureaucratese, particularly common in EU circles)
The person responsible for the formal authorship and presentation of a policy document or report drawn up by a committee, working party, working group, or whatever. This position is standard in French, Belgian and Dutch politics (probably inter alia), but doesn't really exist in British or Irish administrative circles, where parliamentary reports and so on tend to be named for the chairman of the committee; the rapporteur is a quite distinct position and the two appointments may be used as a (simplistic or token) way of levelling out political or geographical bias. As a result, when the UK and Ireland joined the EU the French term was used verbatim in English for the authors of European Parliament committee reports and the opinions issued by the Economic and Social Committee and the Committee of the Regions.
The OED cites a few earlier uses of the word, mainly italicised and in reference to French institutions (back as far as the revolution) and other international organisations such as the League of Nations.