What is the definition of focal infiltrate on a chest x-ray?

Density. Focal density or infiltrate on radiograph is nonspecific term which could represent infection/pneumonia, volume loss/atelectasis, hemorrhage or tumor.
Lung infiltrate. When the air sacs, alveoli, get filled with pus (pneumonia), blood (trauma), cells (cancer) or water (heart failure) they turn white on an X-ray causing a focal infiltrate. Most commonly it is pneumonia.
Pneumonia. A focal infiltrate would be a localized area of the lung that is relatively un-inflated with air compared to the rest of the lung. This can be from pneumonia or atelectasis (lung collapse) in which case there might be a reason for a person not being able to take a normal breath such as being in pain or being unconscious. Sometimes a bronchial tube can be blocked by secretions or mass.