Rosacea is a chronic condition characterized by facial erythema (redness). Pimples are sometimes included as part of the definition.

It is a common but often misunderstood condition that is estimated to affect over 45 million people worldwide. It affects mostly caucasians of mainly north-western European descent, and has been nicknamed the 'curse of the Celts' by some in Britain and Ireland, but can also affect people of other ethnicities. It begins as erythema on the central face and across the cheeks, nose, or forehead but can also less commonly affect the neck, chest, ears, and scalp. As rosacea progresses, other symptoms can develop such as semi-permanent erythema, telangiectasia (dilation of superficial blood vessels on the face), red domed papules (small bumps) and pustules, red gritty eyes, burning and stinging sensations, and in some advanced cases, a red lobulated nose (rhinophyma). The disorder can be confused with, and co-exist with acne vulgaris and/or seborrhoeic dermatitis.

Rosacea affects both sexes, but is almost three times more common in women, and has a peak age of onset between 30 and 60. The presence of rash on the scalp or ears suggests a different or co-existing diagnosis as rosacea is primarily a facial diagnosis, although it may occasionally appear on the scalp or ears.