Longman Dictionary of Contemporary English
Douglas Harper's Etymology Dictionary
1824, from semi- + professional (adj.). As a noun from 1843. Related: Semi-professionally.
Wiktionary
a. 1 (context of a sportsman or athlete English) playing for pay on a part-time basis 2 (context of a team English) composed of semi-professional players n. a semi-professional sportsman or athlete
Wikipedia
A semi-professional athlete is one for whom sport is not a full-time occupation. They are not amateur because they receive regular payment from their team, but at a much lower rate than a full-time professional athlete. As a result, players may have (or seek) a second full-time job. A semipro player/team could also be one that represents a place of employment that only the employees are allowed to play on. In this case, it's considered semipro because their employer pays them, but for their regular job, not for playing on the company's team.
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.
Usage examples of "semi-professional".
It was in the apartment of Anders Bospoort, vice lord and semi-professional rapist, where the greatest anomaly was centred.
He has published about a dozen short stories and articles in professional and semi-professional magazines.
He’s very active in the Quinto Players—it’s a semi-professional theatrical group.