An Acronym with a variety of expansions, all referring to the same thing:

  • Rich Site Summary (RSS 0.91)
  • RDF Site Summary (RSS 0.90, RSS 1.0)
  • Really Simple Syndication (RSS 2.0)
  • no expansion at all (RSS 0.92-0.94)

There are 9 different incompatible versions, but the point is always the same: they're XML vocabularies (and some of them are RDF in XML form) for expressing NewsFeeds.