1. A skin reaction caused by erection of the hair follicles: Piloerection can be caused by cold or shock due to contraction of the "arrector pili" muscles. This causes transient roughness of the skin.
2. An involuntary erection or bristling of hairs due to a sympathetic reflex: Piloerection is usually triggered by cold, shock, or fright, or due to a sympathomimetic agent.

"Goose bumps" (American English), also called "goose pimples", "goose flesh" (British English), "chicken skin" (Hawaiian Pidgin), or "cutis anserina", are the bumps on a person's skin at the base of body hairs which involuntarily develop when a person is cold or experiences strong emotions, such as fear. The reflex of producing goose bumps is known as "horripilation" or "pilomotor reflex". It occurs in humans and in other mammals.

Piloerection starts when a stimulus such as cold or fright causes a discharge from the (involuntary) nervous system that triggers a contraction of the little "arrectores pilorum" muscles. Contraction of these muscles elevates the hair follicles above the rest of the skin so the hair seems to "stand on end".