Eczema is a form of dermatitis, or inflammation of the epidermis (the outer layer of the skin). In England, an estimated 5,773,700 or about one in every nine people have been diagnosed with the disease by a clinician at some point in their lives.

The term eczema is broadly applied to a range of persistent skin conditions. These include dryness and recurring skin rashes that are characterized by one or more of these symptoms: redness, skin edema (swelling), itching and dryness, crusting, flaking, blistering, cracking, oozing, or bleeding. Areas of temporary skin discoloration may appear and are sometimes due to healed injuries. Scratching open a healing lesion may result in scarring and may enlarge the rash.

Eczema may be confused with urticaria. In contrast to psoriasis, eczema is often likely to be found on the flexor aspect of joints.[citation needed]

In some languages, dermatitis and "eczema" are synonymous, while in other languages "dermatitis" implies an acute condition and "eczema" a chronic one. The two conditions are often classified together.

The term eczema refers to a set of clinical characteristics. Classification of the underlying diseases has been haphazard and unsystematic, with many synonyms used to describe the same condition. A type of eczema may be described by location (e.g., hand eczema), by specific appearance (eczema craquele or discoid), or by possible cause (varicose eczema). Further adding to the confusion, many sources use the term eczema and the term for the most common type of eczema (atopic eczema) interchangeably.

The European Academy of Allergology and Clinical Immunology (EAACI) published a position paper in 2001 which simplifies the nomenclature of allergy-related diseases including atopic and allergic contact eczemas. Non-allergic eczemas are not affected by this proposal.

The classification below is ordered by incidence frequency.

Diagnosis of eczema is based mostly on history and physical examination. However, in uncertain cases, skin biopsy may be useful.

There is no known cure for eczema; therefore, treatments aim to control the symptoms by reducing inflammation and relieving itching.

Dermatitis is often treated with corticosteroids. They do not cure eczema, but are highly effective in controlling or suppressing symptoms in most cases. For mild-moderate eczema a weak steroid may be used (e.g. hydrocortisone or desonide), whilst more severe cases require a higher-potency steroid (e.g. clobetasol propionate, fluocinonide). Medium-potency corticosteroids such as clobetasone butyrate (Eumovate), Betamethasone Valerate (Betnovate) or triamcinolone are also available. Generally medical practitioners will prescribe the less potent ones first before trying the more potent ones. In many countries, weak steroids can be purchased 'over the counter' (e.g., hydrocortisone in UK, United States, Germany, Czech Republic, Australia, Iceland), while the more potent ones require a prescription.