Fear is an unpleasant feeling of perceived risk or danger, whether it be real or imagined.
Fear also can be described as a feeling of extreme dislike towards certain conditions, objects or situations such as: fear of darkness, fear of ghosts, etc.
Fear may underlie some phenomena of behavior modification, although these phenomena can be explained without adducing fear as a factor in them.
Furthermore, application of aversive stimuli is also often ineffective in producing change in the behaviour intended to be changed.
Fearing objects or contexts can be learned; in animals this is being studied as fear conditioning, which depends on the emotional circuitry of the brain.
It is one of the basic emotions and is linked heavily to the amygdala neurons.
In fear, one may go through various emotional stages.
A good example of this is the cornered rat, which will try to run away until it is finally cornered by its predator, at which point it will become belligerent and fight back until it either escapes or is captured.
The same goes with most animals.
Humans can become very intimidated by fear; causing them to go along with one's wishes without caring about their own input.
They can also become equally violent, and can even become deadly; it can cause an instinctive reaction to rising adrenaline levels rather than a consciously thought-out decision.