Atrophic skin: Thinning of the skin that results in a depression and often has a wrinkled "cigarette paper" appearance.

Bulla: A fluid-filled spot (vesicle) larger than ¹/₅ inch (5 millimeters) in diameter.

Crust (scab): Dried blood, pus, or skin fluids on the surface of the skin. A crust can form wherever the skin has been damaged.

Cyst: A hollow lump in the skin that has a wall. The central hollow area may contain fluid or solid material.

Erosion: Loss of part or all of the top layers (epidermis) of the skin. Erosions occur when infection, pressure, irritation, or temperature has damaged the skin. They heal without scarring.

Excoriation: A hollowed-out or linear crusted area caused by scratching, rubbing, or picking at the skin.

Lesion: A general term for any abnormal mark or growth on the skin.

Lichenification: Thickened skin that has accentuated skinfolds or creases that appear as deep grooves and wrinkles. Lichenification is produced by prolonged scratching or rubbing.

Macule: A flat, discolored spot of any shape about ¹/₅ inch (5 millimeters) or less in diameter. Freckles, flat moles, port-wine stains, and many rashes are macular. A patch is a large macule.

Nodule: A solid raised area—deeper and easier to feel than a papule—that is usually round. A nodule sometimes appears to form below the surface of the skin and press upward.

Papule: A solid bump about ¹/₅ inch (5 millimeters) or less in diameter. Warts, insect bites, skin tags, and some skin cancers are papules.

Plaque: A flat, raised area or group of small bumps (papules) typically more than ¹/₅ inch (5 millimeters) in diameter.

Pustule: A fluid-filled spot (vesicle) containing pus.

Scales: Areas of heaped-up, dead epidermal cells, producing a flaky, dry patch. Scales occur with psoriasis, seborrheic dermatitis, and many other disorders.

Scar: An area where normal skin has been replaced by fibrous (scar-forming) tissue. Scars form after destruction of some part of the dermis.

Telangiectasia: Dilated blood vessels near the surface of the skin that often have a twisted appearance and that whiten (blanch) when pressure is applied.

Ulcer: Similar to an erosion, only deeper, penetrating at least part of the dermis. The causes are the same as for erosions. They heal with scarring.

Vesicle: A small, fluid-filled spot ¹/₅ inch (5 millimeters) or smaller in diameter. A bulla is a vesicle larger than 5 millimeters in diameter. Herpes zoster (shingles), chickenpox, burns, allergic reactions, and irritations form vesicles and bullae.

Wheal (hive): Swelling in the skin that produces an elevated, soft, spongy area that appears relatively suddenly and then almost always disappears within 24 hours. Wheals are common allergic reactions to drugs, insect bites, or something that touches the skin.