TopoJSON is an extension of GeoJSON that encodes topology. Rather than representing polygons as individual entities, geometries in TopoJSON files are stitched together from shared line segments called arcs. Note how the arcs are numbered in the following definition of Humboldt county.

{"NAME":"Humboldt","STATE_NAME":"California","COUNTY_CD":"12"}},{"type":"Polygon","arcs":[[7,-7,8,9,10]]