Wiktionary
n. A person who works outside traditional academia in the pursuit of truth and knowledge.
Wikipedia
An independent scholar is anyone who conducts scholarly research outside universities and traditional academia.
In the US, a professional association exists for independent scholars: this association is the National Coalition of Independent Scholars. In Canada, the equivalent professional association is the Canadian Academy of Independent Scholars (in association with Simon Fraser University). Similar organizations exist around the world. Membership in a professional association generally entails a degree of post-secondary education and established research.
Usage examples of "independent scholar".
If it had been a question of his own desires alone, he would have preferred the life of the independent scholar to all others -- or else that of a Glass Bead Game player.