An irrational, intense and persistent fear of certain situations, activities, things, animals, or people. The main symptom of this disorder is the excessive and unreasonable desire to avoid the feared stimulus. When the fear is beyond one’s control, and if the fear is interfering with daily life, then a diagnosis under one of the anxiety disorders can be made. This is caused by what are called, neutral, unconditioned, and conditioned stimuli, which trigger either conditioned or unconditioned responses.

An example would be a person who was attacked by a dog (the unconditioned stimulus) would respond with an unconditioned response. When this happens, the unconditioned stimulus of them being attacked by the dog would become conditioned, and to this now conditioned stimulus, they would develop a conditioned response. If the occurrence had enough of an impact on this certain person then they would develop a fear of that dog, or in some cases, an irrational fear of all dogs.