I believe that amateur status was being defined and discussed well prior to the first modern Olympic games (1896) (assuming this the the "Olympics" that you are refering to, not the ancient games). As one example this is an exerpt from
http://en.wikipedia.org/wiki/Amateur_Rowing_Association
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In 1882 the Metropolitan Rowing Association changed its name to the Amateur Rowing Association, having gained additional member clubs from outside London, and began its evolution into the governing body of rowing.
In 1886 the ARA issued General Rules for Regattas. The ARA adopted Henley Royal Regatta's restrictive definition of amateur which not only excluded those who made their living as profession oarsmen but also anyone "who is or has been by trade or employment for wages a mechanic, artisan or labourer."
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Note that the Henley Royal Regatta started in 1839, I believe as an "amateur" event (no fishermen please).
The modern Olympic Games are likely the best known definers of amateurs.
Dave