Fibroadenoma: The most common benign tumor of the breast and the most common breast tumor in women under 30 years of age. Fibroadenomas are usually solitary and may increase in size during pregnancy. The most common signs of fibroadenoma are a painless, rubbery or firm, moveable mass of the breast. The diagnosis of fibroadenoma is made by biopsy. Treatment may involve surgical removal. Fibroadenomas are rare in postmenopausal women. Juvenile fibroadenomas are rapidly growing, benign breast tumors that occur in adolescent females.