n. A modification of a part of a urogenital sinus into a tube or a groove for the discharge of the secretion of the genital or urinary organs, or both; in most mammals, including man, a complete tube from the bladder to the exterior, conveying urine and semen in the male sex, urine only in the female; in some birds, a penial groove for the conveyance of semen only.n. the penis of any animal; in man, the spongy urethra.