rash
Meaning of rash: One word covering an enormous range of skin eruptions, which is why clinicians describe the shape, spread and feel instead.
Definition of rash
A rash is a visible change affecting an area of skin, altering its color, its texture or both. The word describes an appearance rather than a cause, and covers eruptions produced by infection, allergy, drugs, heat, friction, autoimmune disease and long-term skin conditions. Rashes may be flat or raised, dry or blistered, itchy or entirely painless, and may last hours or years.
How is a rash described?
By the individual marks it is built from, and by where those marks sit. A clinician records whether they are flat or raised, whether they hold fluid, how large they are, how sharply they end, and which parts of the body carry them. Distribution matters as much as appearance: a rash on sun-exposed skin, one following the track of a single nerve, one confined to skin folds and one covering the trunk carry different meanings even when the marks themselves look identical.
| Mark | What it looks like | Seen in |
|---|---|---|
| Macule | Flat and colored, cannot be felt | A freckle, an early viral rash |
| Papule | A small solid raised bump | An insect bite reaction |
| Vesicle | A small blister holding clear fluid | Chickenpox, cold sores |
| Pustule | Raised and filled with pus | An acne spot |
| Wheal | A raised swollen patch that comes and goes | Hives |
| Plaque | A broad raised thickened area | A psoriasis patch |
Why does it matter whether a rash fades under pressure?
Because that separates blood inside vessels from blood outside them. Most red rashes take their color from blood in widened surface vessels, and pressing on the skin empties those vessels so the color briefly disappears and then returns. Blood that has leaked into the tissue cannot be moved, so the mark stays exactly as it was. A rash that fails to fade under pressure is therefore recorded as petechiae or purpura rather than as an ordinary red rash, and it is treated as a finding needing an explanation.
Which rashes get mixed up?
Hives and eczema are the classic pair. Hives are raised itchy wheals that shift position and clear within a day, leaving normal skin behind. Dermatitis, the general word for inflamed skin, produces dry scaly patches that stay put for weeks and thicken with scratching. Fungal rashes often clear in the center while spreading at the edge. A drug eruption typically appears across the trunk and spreads outward some days after a new medicine is started. The itch accompanying many of them, pruritus, is no guide to which one is present.
Also known as
Used in a sentence
A widespread rash was noted over the trunk and upper arms.
Good to know
A rash that does not fade when pressed indicates blood outside the vessels rather than within them, which separates petechiae and purpura from the far commoner red rashes.