In essence the title describes what it is. It is a study (usually a paper or 'desk top' study) to determine if a project is viable. This can range from a 'back of the envelope' calculation of costs to a very detailed look at everything ranging from the political climate to environmental considerations.
The phrase "feasibility study" also refers to the outcome fo the activity which is usually a report.
see:
http://en.wikipedia.org/wiki/Feasibility_study
what is a feasibility study
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