Silhouette Sign-the process whereby two substances of the same density which touch each other blur the edges between them

Here in this case the two substances are the pneumonic patch in the lingula and the left cardiac border.

The right heart border is silhouetted out. 
This is caused by a pneumonia affecting middle lobe of the right lung.

Silhouette Sign

A) Normal chest radiograph; B) Q fever pneumonia affecting the right lower and middle lobes. Note the loss of the normal radiographic silhouette (contour) between the affected lung and its right heart border(suggesting affection of middle lobe) as well as between the affected lung and its right diaphragm border(suggesting affection of lower lobe). This phenomenon is called the silhouette sign.