1) An embryonal pediatric tumor of the kidney which may also be seen rarely in adults. The peak incidence of Wilms tumor is between the second and fifth year of life. Microscopically, it is composed of a mixture of cellular elements (blastemal, stromal, and epithelial). The most common sites of metastasis include the regional lymph nodes, lungs, and liver. -- 2004

2) A kidney cancer (tumor) that occurs in children, usually before age 5.