Fear is a negative, short-lasting, high-alert emotion in response to a perceived thrreat, and, like anxiety, it can be measured as a state or trait.
Brene Brown Atlas of the Heart p. 12
Fear arises when we need to respond quickly to physical or psychological danger that is present and imminent. Fear is a rapid-fire emotion, the physicological reaction can sometimes occur before we even realize that we are afraid. The typical responses are fight, flight, or freeze.