Semantic elements represent the geometry of physical world objects. They consist of
- Point (defining points in space),
- Linear (defining linear features), and
- Polygon (defining simple or complex polygons, usually as area boundaries), and
- Relational (which are sometimes used to explain how other elements work together).
Semantic elements vs elements
Elements and semantic elements are often used interchangeably in OSM, but this can cause confusion. The former is a data primitive used to represent the later. Elements are an implementation detail, while semantic elements carry the desired meaning.
Confusion typically arises when looking at areas, which for historical reasons can be implemented either using closed ways or multipolygons. So depending on the context, the term "way" may describe either a linear feature or an area. Specifying "osm way", "closed way", "polygon", or "linear way" might clear things up.