Claustrophobia is the fear of being trapped or closed in. It's estimated to affect up to 7% of the population. It generally causes anxiety which can lead to a panic attack. It can manifest in small rooms, cars, tunnels, basements, elevators, caves, MRIs, and even very crowded areas. Claustrophobics are not afraid of small spaces per se, but often become obsessed about what would happen to them if they were trapped in such a situation.
It's generally diagnosed by having the patient complete a questionnaire about their anxiety in a number of listed situations.
Claustrophobia generally has to be treated with different types of psychotherapy as drug therapy (such as anti-anxiety medications) are generally ineffective.
In the episode Broken, Stomp suffered from severe claustrophobia which left him unable to function in society.