phlegm

Phlegm is the everyday word for the thick mucus that builds up in the airways and chest, the gluey stuff you feel and try to cough up when you have a cold or chest infection. It is essentially airway mucus that has thickened, often because of inflammation or infection.

It is the same family of secretion as ordinary airway mucus but heavier and stickier, frequently mixed with inflammatory cells, dead microbes, and cellular debris that change its color and consistency. People notice it as a rattle in the chest, a feeling of congestion, or the urge to clear the throat.

Phlegm and sputum describe much the same material; the difference is mainly one of register. Phlegm is the lay and general term for the secretion itself, while sputum is the clinical term used when that secretion is coughed out and examined. Thinning it — with good hydration, humidified air, or medicines called mucolytics — can make it easier to clear.