TopoJSON is an extension of GeoJSON that encodes topology. Rather than representing geometries as independent sets of coordinates, TopoJSON represents geometries as an assembly of shared line segments called arcs. Arcs are sequences of points, and geometries are defined as sequences of arcs. This allows for significant reduction in file size, especially for complex geometries with shared boundaries, as common boundaries are only stored once. TopoJSON files are typically used for representing geographic data in a compact and efficient manner, making them suitable for web-based mapping applications and data visualization. They are particularly useful when dealing with large datasets or when bandwidth is a concern. The format supports various geometric types, including points, lines, and polygons, and can also store associated attribute data. TopoJSON is designed to be easily converted back to GeoJSON for compatibility with existing tools and libraries.