TopoJSON is an extension of GeoJSON that encodes topology. Rather than representing geometries as independent sets of coordinates, TopoJSON geometries are stitched together from shared line segments called arcs. Arcs are sequences of points, and geometries are defined as sequences of arcs. This reduces redundancy and file size, making it more efficient for web-based mapping and visualization. TopoJSON files are typically smaller than equivalent GeoJSON files, especially for complex geometries with shared boundaries. This efficiency is crucial for delivering map data quickly over the internet. TopoJSON also allows for topology-aware operations, such as simplification and generalization, which can further reduce file size without sacrificing visual quality. It's widely used in web mapping libraries like D3.js and Leaflet for creating interactive maps and visualizations. The format is particularly well-suited for representing geographic data with shared boundaries, such as administrative regions or river networks.