In the Witcher universe Chaos, or "the force", or "power", or "primordial Chaos", is the power which is harnessed and used by mages. Sources are innately sensitive to this magical force but usually unable to master it. Many go mad. The force is said to have become much more prominent in the world after the Conjunction of the Spheres.
Places where Chaos is strongest and most accessible are known as intersections. Strangely, cats are sensitive to these places. Dragons are as well, but as they are considered magical beasts, this seems less strange.