In medicine, edema is "abnormal fluid accumulation in tissues or body cavities. Most cases of edema are present under the skin in subcutaneous tissue,"(i.e., as extracellular fluid.)  Not all edema is subcutaneous; some of the most critical are in or around organs, such as the eye, lungs, or chest cavity.
Similar terms are:
- Angioedema is "swelling involving the deep dermis, subcutaneous, or submucosal tissues, representing localized edema. Angioedema often occurs in the face, lips, tongue, and larynx."
- Lymphedema is "edema due to obstruction of lymph vessels or disorders of the lymph nodes.
- Myxedema is a "condition characterized by a dry, waxy type of swelling (edema) with abnormal deposits of mucopolysaccharides in the skin and other tissues. It is caused by a deficiency of thyroid hormones. The skin becomes puffy around the eyes and on the cheeks. The face is dull and expressionless with thickened nose and lips."
Extreme cases of edema are called anasarca.
- Diffuse edema, more commonly called peripheral edema or dependent edema.
- Focal edema from angiodema.
- Hydrostatic edema due to heart failure or venous insufficiency.
- Hypoprotenemic edema due to nephrotic syndrome, cirrhosis, protein-energy malnutrition (including Kwashiorkor and marasmus)
- Capillary lead edema including idiopathic edema
- Acute kidney injury or chronic kidney disease
- Medications such as vasodilator agents used for hypertension may cause edema by stimulating sodium reabsorption.
- Obesity may by associated with both lymphedema and edema. The odds ratio of having obesity are about 2 among people who complain of swollen legs. 75% of morbidly obese patients (body mass index >40) may have lymphedema. Obesity may be associated with edema due to obstructive sleep apnea and secondary pulmonary hypertension. 
Edema in one extremity but not the other may suggest a mechanical problem of blood or lymphatic circulation. Doppler ultrasonography may be informative; a number of additional examinations are specific to lymphedema. Most lymphedema is secondary to surgical or wound trauma, and the history will be informative. Venous and lymphadema can coexist.
On physical examination, when the examiner presses on the edematous tissue, the edema is pitting if the skin does not immediately spring back, leaving a depressed area. Not all edema is pitting. Fast recovery of pitting is associated with lower serum albumin levels. If the pitting lasts more than one minute, the serum albumin is probably over 3 (sensitivity=80%; specificity=76%). Presumably this is related to the viscosity of the interstitial fluid thus hypoalbuminemic interstitial fluid can reform more quickly.
Definitive care will address the underlying etiology, but various treatments can be quick interventions. Diuretics are the most important class of drugs. When the edema builds from ineffective heart pumping as in heart failure, positive inotropic agents help.
When fluid load overcomes the lungs, rotating tourniquets or even therapeutic venepuncture may be lifesaving in severe pulmonary edema.
- Anonymous (2021), Edema (English). Medical Subject Headings. U.S. National Library of Medicine.
- Little RC, Ginsburg JM (August 1984). "The physiologic basis for clinical edema". Archives of internal medicine 144 (8): 1661–4. PMID 6466021.
- Cho S, Atwood JE (November 2002). "Peripheral edema". The American journal of medicine 113 (7): 580–6. DOI:10.1016/S0002-9343(02)01322-0. PMID 12459405. Research Blogging.
- Anonymous (2021), Angioedema (English). Medical Subject Headings. U.S. National Library of Medicine.
- Anonymous (2021), Lymphedema (English). Medical Subject Headings. U.S. National Library of Medicine.
- Anonymous (2021), Myxedema (English). Medical Subject Headings. U.S. National Library of Medicine.
- Peytremann-Bridevaux I, Santos-Eggimann B (2008). "Health correlates of overweight and obesity in adults aged 50 years and over: results from the Survey of Health, Ageing and Retirement in Europe (SHARE). Obesity and health in Europeans aged > or = 50 years.". Swiss Med Wkly 138 (17-18): 261-6. DOI:2008/17/smw-12067. PMID 18481232. Research Blogging.
- Fife CE, Carter MJ (January 2008). "Lymphedema in the morbidly obese patient: unique challenges in a unique population". Ostomy/wound management 54 (1): 44–56. PMID 18250486.
- Blankfield RP, Hudgel DW, Tapolyai AA, Zyzanski SJ (2000). "Bilateral leg edema, obesity, pulmonary hypertension, and obstructive sleep apnea". Archives of internal medicine 160 (15): 2357–62. PMID 10927734.
- Blankfield RP, Zyzanski SJ (June 2002). "Bilateral leg edema, pulmonary hypertension, and obstructive sleep apnea: a cross-sectional study". The Journal of family practice 51 (6): 561–4. PMID 12100781.
- Blankfield RP, Finkelhor RS, Alexander JJ, et al (September 1998). "Etiology and diagnosis of bilateral leg edema in primary care". The American journal of medicine 105 (3): 192–7. DOI:10.1016/S0002-9343(98)00235-6. PMID 9753021. Research Blogging.
- Henry JA, Altmann P (April 1978). "Assessment of hypoproteinaemic oedema: a simple physical sign". British medical journal 1 (6117): 890–1. PMID 638510. PMC 1603695. PubMed Central