A well-defined generally circular (sometimes linear) hole in a thin layer of supercooled water droplet cloud. Virga or wisps of Cirrus typically fall from the central part of the hole, which generally grows larger with time. Cavum is typically a circular feature when viewed from directly beneath, but may appear oval shaped when viewed from a distance.
When resulting directly from the interaction of an aircraft with the cloud, it is generally linear (in the form of a dissipation trail). Virga typically falls from the progressively widening dissipation trail.