The term probiotics means "for life." It was defined by a group of experts convened by the Food and Agriculture Organization (FAO) of the United Nations as "microorganisms administered in adequate amounts which confer a beneficial health effect on the host."

Probiotics are carefully selected for their ability to tolerate the adverse conditions of the gastrointestinal tract such as high acidic environment and high bile concentrations. Probiotics colonize the human body. These bacteria play a significant role in human physiology. For example, probiotics have been shown to lower the pH of the intestinal tract so that pathogenic bacteria cannot survive. In addition, growing beneficial bacteria compete with pathogens for nutrients and space in the intestinal tract. Probiotics also are associated with preventing diarrhea, relieving constipation, building immunity, lowering blood cholesterol, preventing skin allergy and improving women's urogenital health.  Probiotics have been used to treat or prevent diarrhoea and to improve symptoms in lactose intolerance and have also been reported to have anti-carcinogenic properties.

Since probiotics do not permanently colonize the host, they need to be ingested regularly for any health promoting properties to persist.