Halitosis, colloquially called bad breath, feter oris, or fege bosta, is a symptom in which a noticeably unpleasant odor is present on the exhaled breath. Concern about halitosis is estimated to be the third most frequent reason for people to seek dental care, following tooth decay and gum disease; and about 20% of the general population are reported to suffer from it to some degree.

Not all who think they have halitosis have a genuine problem. Of those who feel they have halitosis, significant percentages (5‑72%) have been reported to have no genuine halitosis when professionally examined. Of those who have genuine halitosis, often the odor is caused by bacteria present below the gumline and on the back of the tongue. The remaining 10% is accounted for by many different conditions, including disorders in the nasal cavity, sinuses, throat, lungs, esophagus, stomach or elsewhere. See the related article on tonsilloliths for another possible cause of halitosis.

Very rarely, halitosis can be one of many symptoms of a serious underlying medical condition such as liver failure; but, in the vast majority of cases, the cause is minor and can often be reduced by adjustments to oral hygiene, including brushing or gently scraping the back of the tongue and improving the health of the gums by using dental floss. Occasionally, however, especially if the origin of the odor is not in the mouth, halitosis can be more difficult to diagnose and to manage successfully. Bad breath is a social taboo; and, as a result, perceived or genuine halitosis can sometimes trigger social anxiety and depression.