A lung opacity is a frequently used term by radiologists on chest X-rays and essentially means a white spot of uncertain significance. The lungs are normally black on a chest X-ray so anything that blocks the X-rays from getting through will look white on an X-ray. A white spot amidst the normal black lungs can therefore be a cancer, infection, bleeding, fluid, foreign body amongst other possibilities.
The radiologist who interprets the X-ray tries to provide as accurate and specific diagnosis as he can using the clinical information given, such as cough and fever, prior studies, and laboratory information. Therefore, an opacity on a chest X-ray in a patient with cough and fever presenting to the emergency room will most likely be a pneumonia. A more rounded opacity in a long time smoker is more likely to be a cancer. White opacities in both lungs in someone known to have heart failure is most likely edema or fluid in the lungs.
Therefore the radiologist also uses the pattern of abnormality or opacity to determine the most likely diagnosis. If it is in one small area then it may be a lung nodule. A larger abnormality can be a pneumonia or lung collapse. A round abnormality can be a mass or cancer. An abnormality on both sides of the lung can be many things like a bad pneumonia, fluid, or chronic lung disease. The clinical information is helpful as are prior exams.
If an opacity is new compared to say something recent from a month ago, then it is more likely to an infection or fluid. A growing opacity is more likely to be cancerous. A stable opacity over time and many X-rays is more likely to be something benign, perhaps scarring from prior infection. A slowly worsening abnormality in both lungs may represent chronic lung disease.
So in a way, lung opacity is the least differentiated way of saying something is wrong in the lung but hard to say what it is based on all the information present. A lung opacity can be something benign like scarring all the way to deadly cancer. Often times, the radiologist interpreting the X-ray will provide a differential diagnosis which is a list of possibilities. He will also recommend what to do in some cases. Like get a follow up or get a CT scan to take a closer look and see if a more precise diagnosis can be made.
Therefore, if the radiologist describes an abnormality in the lung as an opacity, rather then a specific diagnosis, then this may mean that more testing and more information is needed to reach the correct diagnosis. Your clinical doctor will use the information from the X-ray together with everything he knows about you as a patient to arrive at the best course of action and diagnosis.