Kidney stone: A stone in the kidney or a stone that originates in the kidney but has passed lower down in the urinary tract. Kidney stones are a common cause of blood in the urine and pain in the abdomen, flank, or groin. The development of kidney stones is related to decreased urine volume or to increased excretion of stone-forming components, such as calcium, oxalate, urate, cystine, xanthine, and phosphate. The stones form in the urine-collecting area (pelvis) of the kidney and may range in size from tiny to 'staghorn' stones the size of the renal pelvis itself. Factors that predispose people to kidney stones include reduction in fluid intake, increased exercise with dehydration, medications that cause high uric acid (hyperuricemia), and a history of gout. Pain from kidney stones is usually of sudden onset, very severe and intermittent, and not improved by changes in position, and it radiates from the back, down the flank, and into the groin. Nausea and vomiting are common. The majority of stones pass spontaneously within 48 hours. However, some stones do not. Several factors influence the ability to pass a stone, including the size of the person, prior stone passage, prostate enlargement, pregnancy, and the size of the stone. If a stone does not pass, the help of a urology specialist may be needed. Routine treatment includes relief of pain, hydration, and, if there is concurrent urinary infection, administration of antibiotics. Also known as nephrolithiasis.