Area/The Future of Areas/Simple Features
Simple Features is a standard way of modelling geographic data (such as points, lines, polygons, etc.).
See the wikipedia article for links to the specs.
OSM and Simple Features
Simple Features is a widely deployed standard in the GIS industry. Almost all software that handles spatial data uses Simple Features as a data model. OSM uses a different data model which has advantages and disadvantages when compared to the Simple Features model.
OSM will probably never switch to the Simple Features model, but it makes sense to think about how those two models are different and where we can use ideas from it. We also have to define a mapping between OSM and Simple Features (in both directions), because so many software uses the Simple Features model. For nodes (that map to a Simple Feature "Point") and ways (that map to a Simple Feature "LineString") this is straightforward. For areas/polygons this is much more difficult. This has hampered the use of OSM with software that is using the Simple Feature model.
Mapnik rendering is based on the Simple Features model. Osm2pgsql is converting native OSM data into PostGIS database and the resulting tables osm_point, osm_line and osm_polygon are all filled with Simple Features.
The simple feature model defines certain words in certain ways. To make discussions easier, we should use those words with those meanings and use different words for OSM concepts that might be similar but don't match the definitions.
Please read up on those definitions in the standard. Maybe somebody wants to copy the most important parts here.