Fluid from vessels. Usually a more or less protein rich fluid leaking out of blood vessels in the lungs depending on the cause.
Serum. Cardiac or capillary leak etiologies result in serum accumulation in the lungs in pulmonary edema (greek for swelling). In essence, affected patients are drowning in their own serum when pulmonary edema sets in and can rapidly die or suffer permanent brain damage unless this is emergently and expertly treated with diuretics and cardiotonics while attempts to reverse the inciting cause are underway.