Extreme and irrational fear of a particular object, class of objects, or situation. A phobia is classified as a type of anxiety disorder (a neurosis), since anxiety is its chief symptom. Phobias are generally believed to result when fear produced by an original threatening situation (such as a near-drowning in childhood) is transferred to other similar situations (such as encounters with bodies of water), the original fear often being repressed or forgotten. Behaviour therapy can be helpful in overcoming phobias, the phobic person being gradually exposed to the anxiety-provoking object or situation in a way that demonstrates that no threat really exists.
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Phobia is also used in a non-medical sense for aversions of all sorts. These terms are usually constructed with the suffix -phobia. A number of these terms describe negative attitudes or prejudices towards the named subjects. See Non-clinical uses of the term below.
Phobias are more often than not linked to the amygdala, an area of the brain located behind the pituitary gland in the limbic system. The amygdala secretes hormones that control fear and aggression, and aids in the interpretation of this emotion in the facial expressions of others. When the fear or aggression response is initiated, the amygdala releases hormones into the body to put the human body into an "alert" state, in which they are ready to move, run, fight, etc. This defensive "alert" state and response is generally referred to in psychology as the Fight-or-flight response.

According to the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV), social phobia, specific phobia, and agoraphobia are sub-groups of anxiety disorder.
Many of the specific phobias, such as fear of dogs, heights, spiders and so forth, are extensions of fears that a lot of people have. People with these phobias specifically avoid the entity they fear.
Phobias vary in severity among individuals. Some individuals can simply avoid the subject of their fear and suffer only relatively mild anxiety over that fear. Others suffer fully-fledged panic attacks with all the associated disabling symptoms. Most individuals understand that they are suffering from an irrational fear, but are powerless to override their initial panic reaction.
Severe fears are present in about 10-15% of children and specific phobias are found in about 5% of children. Children with specific phobias experience an intense fear of an object or situation that does not go away easily and continues for an extended period of time. Children often have specific phobias of the dark, varieties of insects, spiders, bees, heights, water, choking, snakes, dogs, birds, and other animals. For many children, these fears and phobias interfere with their participation in and enjoyment of various activities. It may also interfere with their education, family life, or their social life. However, effective treatment is available for children who experience phobias.
Cognitive behavioral therapy (CBT) can be beneficial. Cognitive behavioral therapy lets the patient understand the cycle of negative thought patterns, and ways to change these thought patterns. CBT may be conducted in a group setting. Gradual desensitisation treatment and CBT are often successful, provided the patient is willing to endure some discomfort and to make a continuous effort over a long period of time.
Hypnotherapy coupled with Neuro-linguistic programming can also be used to help remove the associations that trigger a phobic reaction.
Anti-anxiety or anti-depression medications can be of assistance in many cases. Benzodiazepines could be prescribed for short-term use.
Emotional Freedom Technique, a psychotherapeutic alternative medicine tool, considered to be pseudoscience by the mainstream medicine, is allegedly useful.
These treatment options are not mutually exclusive. Often a therapist will suggest multiple treatments.
A number of terms with the suffix -phobia are primarily understood as negative attitudes towards certain categories of people or other things, used in an analogy with the medical usage of the term. Usually these kinds of "phobias" are described as fear, dislike, disapproval, prejudice, hatred, discrimination, or hostility towards the object of the "phobia". Often this attitude is based on prejudices and is a particular case of general xenophobia.
Class discrimination is not always considered a phobia in the clinical sense because it is believed to be only a symptom of other psychological issues, or the result of ignorance, or of political or social beliefs. In other words, unlike clinical phobias, which are usually qualified with disabling fear, class discrimination usually has roots in social relations.
Below are some examples: