Regurgitation is blood flow in the opposite direction from normal, as the backward flowing of blood into the heart or between heart chambers. It is the circulatory equivalent of backflow in engineered systems. It is sometimes called reflux.

Regurgitation in or near the heart is often caused by valvular insufficiency (insufficient function, with incomplete closure, of the heart valves); for example, aortic valve insufficiency causes regurgitation through that valve, called aortic regurgitation, and the terms aortic insufficiency and aortic regurgitation are so closely linked as usually to be treated as metonymically interchangeable.

The various types of heart valve regurgitation via insufficiency are as follows:

1. Aortic regurgitation: the backflow of blood from the aorta into the left ventricle, owing to insufficiency of the aortic semilunar valve; it may be chronic or acute.
2. Mitral regurgitation: the backflow of blood from the left ventricle into the left atrium, owing to insufficiency of the mitral valve; it may be acute or chronic, and is usually due to mitral valve prolapse, rheumatic heart disease, or a complication of cardiac dilatation. See also Mitral regurgitation.
3. Pulmonic regurgitation: the backflow of blood from the pulmonary artery into the right ventricle, owing to insufficiency of the pulmonic semilunar valve.
4. Tricuspid regurgitation: the backflow of blood from the right ventricle into the right atrium, owing to imperfect functioning (insufficiency) of the tricuspid valve.