Folk high schools (Danish: Folkehøjskole; Finnish: kansanopisto and työväenopisto or kansalaisopisto; German: Volkshochschule and (uncommon) Heimvolkshochschule; Norwegian: Folkehøgskole; Swedish: Folkhögskola) are institutions for adult education that generally do not grant academic degrees, though certain courses might exist leading to that goal. They are most commonly found in Nordic countries and in Germany and Austria. The concept originally came from the Danish writer, poet, philosopher and pastor Nikolaj Frederik Severin Grundtvig (1783–1872). Grundtvig was inspired by the Marquis de Condorcet's Report on the General Organization of Public Instruction which was written in 1792 during the French Revolution. The Revolution had a direct influence on popular education in France. In the United States, a Danish folk school called Danebod was founded in Tyler, Minnesota.

Despite similar names and somewhat similar goals, the institutions are quite different in Germany and Sweden as opposed to the traditions in Denmark and Norway. Folk high schools in Germany and Sweden are in fact much closer to the institutions known as folkeuniversitet or folkuniversitet in Norway and Denmark which provide adult education. However, unlike the folkuniversitet, folk high schools in Sweden are not connected to a regular university. The Finnish työväenopisto or kansalaisopisto (called arbetarinstitut in Swedish) are also part of the educational Folk tradition.

High school is a term used in parts of the English-speaking world to describe institutions which provide all or part of secondary education, but not always the highest years of basic education, which can be called a "secondary school" or "secondary college" or other terms, depending on the nation or region (see below: Usage by country). The phrase "high school" is often incorporated into the name of the related institutions.

The term (as "high school") originated in Scotland, with the world's oldest being Edinburgh's Royal High School from 1505. The Royal High School was used as a model for the first public high school in the United States, the English High School founded in Boston, Massachusetts in 1821. The precise stage of schooling provided by a high school differs from country to country, and may vary within the same jurisdiction. In all of New Zealand and Malaysia along with parts of Australia, Bangladesh and Canada, high school is synonymous with secondary school, and encompasses the entire secondary stage of education.