Goose bumps are small bumps in the skin that are caused by tightening muscles, which pull body hair into an erect position. In humans, these bumps are a vestigial reflex, left over from a time when humans had more hair; a wide variety of animals demonstrate this interesting reflex in response to certain stimuli. Many people have experienced goose bumps at some point in their lives, and some may have wondered why they appear.
There are two main reasons why goose bumps manifest: cold and fear. In animals that actually have fur, standing individual hairs on end in cold weather is a sensible course of action, since the erect hairs can trap air, adding a layer of insulation to protect the animal from the cold. In the case of fear, goose bumps in an animal with fur would cause the animal to look larger, potentially intimidating the animal that is threatening it, giving the animal a chance to run or fight.
The medical term for this reaction is cutis anserina, and the term “horripilation” is sometimes used to refer to the act of raising goose bumps. This reflex is part of the body's “fight or flight” system, meaning that it is entirely involuntary. It is triggered by a signal from the autonomic nervous system, which tells the muscles around the hair follicles to tighten, raising the hairs and creating a distinctive tight lump. They are common on the arms and legs, but goosebumps can technically appear anywhere, including on the face, scalp, and chest.
A number of colorful slang terms are used to refer to goose bumps, including gooseflesh, goose pimples, chill bumps, and chicken skin. The abundance of avian-related slang is a reference to the skin of plucked geese and chickens, which often has a similar raised pattern. Because birds don't actually have hair, technically geese don't get goose bumps.
As a general rule, goose bumps are not a cause for alarm, although the situation which is causing them could be a potential source of worry. However, sometimes they are associated with certain medical conditions, particularly those involving the brain, so someone who gets them a lot or for no apparent reason may want to seek medical attention. They are also common in people experiencing drug withdrawal.