on·col·o·gy 
(ŏn-kŏlə-jē, ŏng-)
(ŏn-kŏlə-jē, ŏng-)n.
The branch of medicine that deals with tumors, including study of their development, diagnosis, treatment, and prevention.
[Greek onkos, mass, tumor; see nek-3 in the Appendix of Indo-European roots + -LOGY.]
on′co·logi·cal (-kə-lŏjĭ-kəl), on′co·logic (-lŏjĭk) adj.
on·colo·gist n.