Basically, an infiltrate is an ill-defined shadow in the lung
, on chest x-ray, with features best illustrated in the shadows of pneumonia
. That doesn't mean all infiltrates are pneumonia. They are not. But, the type of shadow that typifies infectious pneumonia can also be a sign of non-infected fluid or blood, dead lung tissue, collapse of a portion of lung, non-infectious inflammation as with aspiration of stomach acid, or even tumor.