It is a clinical syndrome wherein heart fails to pump blood at a rate required by the tissues of the body or it can do so only with an elevated filling pressure. This may be due to insufficient cardiac filling or impaired contractility and emptying. Various compensatory mechanisms set in to increase blood volume, heart rate and cardiac muscle mass. Despite these compensatory mechanisms, the ability of the heart to contract and relax may decline leading to worsening heart failure. 

An alternative definition stresses on the clinical consequences of heart failure. Heart failure has been defined as a complex clinical syndrome i.e. characterized by abnormal left ventricular function and neurohormonal regulation, leading to effort intolerance, fluid retention and reduced longevity.

Other terms like "myocardial failure" and "circulatory failure" should be differentiated from the term "heart failure". Myocardial failure denotes abnormal systolic and diastolic function of the heart, which may be symptomatic or asymptomatic. Circulatory failure can be due to noncardiac conditions like haemorrhagic shock, while cardiac function is preserved.