From OpenStreetMap Wiki
- Originally they were a separate data primitive alongside nodes and ways. These were removed in October 2007, to simplify the data model, with the introduction of protocol version 0.5. Before this, the following was true: Segments connect two nodes. Segments should be always connected to a way or an area (if not they were are “unwayed segments”). One segment can be a part of several ways or areas. Segments can be used alone (eg. not as part of a Way or Area) but this is not recommended. If segments are applied without ways or areas, these must have at least one "key" with a valid "value".
- Later on (after October 2007), the word "Segment" is increasingly being used to mean, a straight section of a Way between exactly two nodes. This is similar to its previous meaning, but a segment manifests itself within node and way definitions rather than being an actual data primitive. To be clearer it may be better to refer to them as "way-segments".