Prostate: A gland in the male reproductive system. The prostate surrounds the part of the urethra (the tube that empties the bladder) just below the bladder, and produces a fluid that forms part of the semen. It lies behind the symphysis pubis (to which it is connected by the puboprostatic ligament), above the urogenital diaphragm (pelvic floor), and in front of the rectum. The adult prostate weighs about 20 grams and is typically divided into three lobes: right and left lateral lobes and the middle lobe. These lobes consist of alveoli lined with columnar epithelium encased in a think capsule of connective tissue and embedded in a thick fibromuscular stroma. The prostate secretes an alkaline substance which constitutes the largest part (60 percent) of the seminal fluid.