A small, hard deposit that forms in the kidneys and is often painful when passed. Urolithiasis is a term used to describe calculi or stones that form the urinary tract. Urolithiasis: The process of forming stones in the kidney, bladder, and/or urethra ( urinary tract). Urolithiasis -or else called Urinary Calculosis- is the formation of stones (calculi) anywhere in the urinary tract (in the renal pyelocalyceal system, ureter, bladder,. Kidney stones (also called renal calculi, nephrolithiasis or urolithiasis) are hard deposits made of minerals and salts that form inside your kidneys. Renal calculi are formed when the urine is supersaturated with salt and minerals such as calcium oxalate, struvite (ammonium magnesium.