Nephroptosis: A condition in which the kidney descends more than the length of 2 vertebral bodies (or more than 5 cm) when a person moves from a lying down to upright position. Its cause is unknown, but it often occurs in young, thin women. When symptoms are present, these include lower abdominal or flank pain that is worsened by prolonged standing and relieved by lying down. It can also cause symptoms of vomiting and acute abdominal pain due to acute obstruction or ischemia of the kidney. Also known as floating kidney or rental ptosis.