In medicine, the term "shiver" refers to involuntary muscle contractions or trembling movements that occur as a physiological response to various stimuli. Shivering is often associated with feeling cold, as it is the body's natural mechanism to generate heat and increase body temperature. It involves rapid contraction and relaxation of muscles, which helps to generate heat through increased metabolic activity. Shivering can also be caused by other factors such as fever, stress, fear, certain medications, or underlying medical conditions.