Edema is the accumulation of fluid in bodily tissues or a body cavity. The word edema is derived from the Middle English word ydema, which comes from the Greek word for "swelling." Edema is an observable condition in which the swelling is usually apparent. Most commonly found in the feet and legs, edema occurs beneath the skin in the spaces within the body tissues outside of the blood vessels, called interstitial spaces or compartments.

Edema is most often a manifestation of underlying diseases of the liver, heart and kidneys and is directly caused by salt retention. Besides the legs and feet, other main areas in which edema can occur are the lungs (alveoli), the abdomen or peritoneal cavity and the chest. When edema occurs in the lungs, it is called pulmonary edema and is usually caused by heart failure.

Most people can take in a large amount of salt without complication, but those with underlying disease have a problem processing and eliminating excess salt. Those with heart disease experience a reduced flow of blood to their kidneys, which then affects the kidneys' ability to eliminate the excess. This leads to the retention of fluid, and possibly edema.

There are two types of edema, which can help a doctor diagnose the cause: pitting and non-pitting. With pitting edema, the swollen area shows indentations from the socks or stockings. Non-pitting edema does not retain an indentation and is often seen in people with lymphatic system problems or hypothyroidism.

Idiopathic edema is a type of pitting edema which occurs without any obvious underlying disease or condition. It is most often seen in women who experience edema in the legs and feet when they are pre-menopausal or pre-menstrual. The condition is therefore also known as cyclical edema. Besides treating the underlying condition that contributes to edema, doctors often recommend diuretics and reduction of salt intake to treat the swelling itself.