sputum

This term is primarily used in clinical and medical settings to describe the substance expelled from the lower airways. It is distinguished from simple saliva, which is produced in the mouth, by the presence of mucus and cellular debris from the lungs. As a mass noun, sputum is treated as uncountable. It does not have a plural form in standard medical usage; instead, clinicians refer to a sputum sample or a volume of sputum when quantifying the substance.