Fear

Fear is an intensely unpleasant emotion in response to perceiving or recognising a danger or threat. Fear causes physiological changes that may produce behavioural actions such an aggressive response or fleeing from threat. 

The fear response arises from the perception of danger. This can cause a person to confront or avoid the threat, or which in some cases can be a freeze response. 

An irrational fear is called a phobia. Fear is also related to the emotion anxiety, which occurs as the result of threats that are perceived to be uncontrollable or unavoidable.