Technically: the definition of the star cluster you're playing in: the names and locations of all the planets.
Generally, you see the whole cluster (i.e. all planet positions) from the start. The popular add-on ExploreMap changes this so you only see planets in scan range.
The starchart consists of the files:
- xyplan.dat: planet positions. In Explore Map games, each player has his own xyplanN.dat file.
- planet.nm: the planet names.
Of course, the term starchart is also applied to the section of a client program where you can browse through the universe, compiled from the above files and usually much more information.
