Claustrophobia is NOT the fear of elevators. Claustrophobia is defined by dictionary.com as "an abnormal fear of being in enclosed or narrow places". This simple definition allows us to see why the term has been associated with elevators and MRI machines. It is perfectly normal to be apprehensive about small spaces, but if a person becomes unusually frightend in such situations they are described as being claustrophobic.
Claustrophobic reactions range from mild reactions like increased sweating and heart rate to more severe reactions such as nausea or even fainting. In some cases claustrophobic patients can have these symptoms occur by just thinking about being in a constricted environment. A great informational article about claustrophobia can be found by clicking the following link: Epigee.org