A phobia is an irrational or exaggerated fear of a particular object or situation. In clinical psychology, specific phobias involve extreme anxiety about one particular object, while social phobias involve a more generalised anxiety about a range of social situations.
Triskaidekaphobia is an excessive fear of the number 13. As this fear is often enshrined in the superstitious belief system of many cultures, it is probably not justifiable to view it as an anxiety disorder.
Because the word phobia is Greek, phobias are named using Greek prefixes. On occasions this gives rise to some extraordinary-sounding creations: for example, a fear of the day/date combination Friday, the 13th is known as paraskevidekatriaphobia or friggatriskaidekaphobia.