What is the meaning of an "ill-defined opacity of the lung"?

In a chest X-ray, an ill-defined opacity of the lung refers to an area of the lung that is shaded rather than translucent. It does not have the clearly defined edges of an anatomical structure such as the heart, according to Health Hype.

In a chest X-ray, viewing the lungs from the prone side, the heart and ribs form defined opacities behind or around the lungs, and the lungs are translucent, explains Health Hype. An ill-defined opacity is a shaded area that does not have the qualities of these normal internal structures, but instead is often an indication of fluid, nodules, infection, swelling or other abnormalities in the lung tissue itself.