Edema is the excessive fluid retention inside a tissue space, which we call edema, where the tissue space often refers to the skin or subcutaneous tissues. As for fluid retention inside body cavities, we call it effusion, i.e., for example, pleural and abdominal effusion. Edema can be classified as mild, moderate, or severe. Mild edema is often seen in the eyelids, orbits, or ankles, or anterior tibiae, etc. It is called mild edema. Moderate edema is often seen in loose tissue areas throughout the body. Severe edema refers to this severe edema throughout the body, often accompanied by pleural effusion, abdominal effusion, or pericardial effusion. There are many causes of edema, such as renal edema, cardiac edema, hepatic edema, malnutrition, endocrine causes and so on.