Bell's palsy: Paralysis of the nerve that supplies the facial muscles on one side of the face (the seventh cranial nerve, or facial nerve). Bell's palsy often starts suddenly. The cause may be a viral infection. Treatment includes protecting the eye on the affected side from dryness during sleep. Massage of affected muscles can reduce soreness. Sometimes cortisone medication, such as prednisone, is given to reduce inflammation during the first weeks of illness. The outlook is generally good; the vast majority of patients recover within weeks or months.