Urea:

the major nitrogen-containing product of protein catabolism.
a nitrogen-containing waste product from the breakdown of proteins that is excreted in the urine. Urea is produced in the liver by the condensation of two molecules of ammonia and one molecule of carbon dioxide. 
a waste product found in the blood that results from the normal breakdown of protein in the liver. Urea is normally removed from the blood by the kidneys and then excreted in the urine.