Mouth breathing is breathing through the mouth rather than the nose.
Human infants are sometimes considered obligate nasal breathers, but generally speaking healthy humans may breathe through their nose, their mouth, or both. During rest, breathing through the nose is common for most individuals. Breathing through both nose and mouth during exercise is also normal, a behavioral adaptation to increase air intake and hence supply more oxygen to the muscles. Mouth breathing may be called abnormal when an individual breathes through the mouth even during rest. Some sources use the term "mouth breathing habit" but this incorrectly implies that the individual is fully capable of normal nasal breathing, and is breathing through their mouth out of preference. However, in about 85% of cases, mouth breathing represents an involuntary, subconscious adaptation to reduced openness of the nasal airway, and mouth breathing is a requirement simply in order to get enough air. Chronic mouth breathing in children may affect dental and facial growth. It may also cause gingivitis (inflamed gums) and halitosis (bad breath), especially upon waking if mouth breathing occurs during sleep.
The term "mouth-breather" is sometimes utilized as an insult to imply low intelligence.