Prostate cancer: A malignant tumor of the prostate, the gland that produces some of the components of semen. Prostate cancer is the second leading cause of death of males in the US. It is often first detected as a hard nodule found during a routine rectal examination. The PSA blood test is a screening test for prostate cancer. Diagnosis of prostate cancer is established when cancer cells are identified in prostate tissue obtained via biopsy. In some patients, prostate cancer is life threatening. In many others, prostate cancer can exist for years without causing any health problems. Treatment options for prostate cancer include observation, radiation therapy, surgery, hormone therapy, and chemotherapy.