Prickly heat (miliaria)

Miliaria, commonly known as heat rash, sweat rash, or prickly heat, is a skin disease marked by small, itchy rashes due to sweat trapped under the skin by clogged sweat-gland ducts. Similar to sunburn, miliaria is common in hot and humid conditions, such as in the tropics and during the summer. Although it affects people of all ages, it is especially common in children and infants due to their underdeveloped sweat glands.

Underlined words are explained — tap any of them.