Definition: Fluid accumulates in pleural space.

You are looking at pleural effusion on right, in the Chest x-ray.

Irrespective of the nature of fluid, radiologically they will look similar. 

Radiological criteria are:

• Homogenous density
• Density in dependent portion
  • Upright: Costophrenic angle in PA view 
  • Lateral view: Anterior and posterior portions of gutter 
  • Lateral decubitus position: Along sides 
  • Supine position: Along posteriorly, giving diffuse haziness on the side of effusion
• Silouhette of upper limit of density
  • Upper margin high in axilla in PA view
  • Upper margin high anteriorly and posteriorly in lateral view
  • This is just an illusion
• Loss of silouhette. Note lack of identifiable left diaphragm before and visible diaphragm after clearance of fluid (Silouhette sign principle).