The term "cotton-like density" isn't exclusive to one condition as it is a statement regarding the appearance of a disease process. It occurs when there is obliteration of the lung parenchymal architecture due to the infiltration of cells or fluid (remember that exudate and hemorrhage are essentially fluid). Unlike air bronchograms even the airways (bronchioles) in the region are obliterated or flooded. Radiographically it appears as relatively discrete areas of opacity with either discrete or non-discrete borders. Typically, a cotton-like density will be radiopaque with a "fluffy" border. Cotton-like densities may be found in individuals with: pulmonary osteomas, neoplasia, fungal infiltrates and areas of abscessation.
Cotton-like Densities - example 1
Cotton-like Densities - example 2