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A semi-professional athlete is one who is paid to play and thus is not an amateur, but for whom sport is not a full-time occupation, generally because the level of pay is too low to make a reasonable living based solely upon that source, thus making the athlete not a full professional athlete.
Likewise the term semi-professional can be applied to an artist such as a photographer or musician who derives some income from their artistic endeavors but who must nevertheless take a day job in order to survive. When applied to vocational tools and equipment, it refers to products that lie between the amateur and professional levels in both quality and cost, though nowadays the term prosumer is often used instead.
The San Francisco Olympic Club fielded a football team in 1890. That year, the Olympic Club was accused by a rival club of enticing athletes to jump to its ranks with offers of jobs. An investigation by the Amateur Athletic Union ruled that the Olympics' practice was not actually professionalism but only a "semi" form of it, thus inventing the term "semi-pro". Although the Amateur Athletic Union didn't like the idea very much, it decided that clubs could indeed offer employment without losing their amateur status or compromising the athlete.
North America 
In North America, semi-professional athletes and teams were far more common in the early and mid-20th century than they are today. There are many benefits, such as collegiate eligibility and the attendant scholarships, in maintaining amateur status. Eligibility for participation in the Olympics in some sports is still dependent upon maintaining a purely amateur status (although far less so than was previously the case), and such athletes may be supported by government money, business sponsorships, and other systems. At the same time, professional sports have become such a massive and remunerative business that even many low-level feeder teams can afford to have fully professional athletes.
Semi-professionalism is most prevalent in junior ice hockey, in which the top levels of Canadian junior hockey (most of whom are teenagers still in, or just out of, high school) are paid at a semi-professional level. This is not the case in the United States, where college ice hockey dominates at that age group; the junior leagues in the United States thus generally operate as fully amateur teams to maintain the players' eligibility to play in college.
United Kingdom 
There are many semi-professional football teams at non-League level. The bottom division of The Football League (the fourth tier of the English football league system) has traditionally been the cut-off between professional ("full-time") and semi-professional ("part-time") in English football. However, many teams in the top non-League competition, the Conference National have become "full-time" in an effort to achieve League status.
Women's football in England is semi-professional at the top levels, as finances depend both on promotion and relegation of parent male teams as of the female teams themselves. Full professionalism for women is thus still in the planning stages; top female players often depend on other sources of income (such as coaching and physical training), and many attend university or college while playing.
Historically, English rugby league and rugby union have had one full-time professional division, with semi-professional divisions at the next level down. The second tier of union, the RFU Championship, became fully professional beginning with the 2009–10 season.