A secondary school or high school is an institution that provides secondary education. Some secondary schools provide both lower secondary education and upper secondary education, i.e., both levels 2 and 3 of the ISCED scale, but these can also be provided in separate schools.

Secondary education or post-primary education covers two phases on the International Standard Classification of Education scale. Level 2 or lower secondary education is considered the second and final phase of basic education, and level 3 upper secondary education or senior secondary education is the stage before tertiary education. Every country aims to provide basic education, but the systems and terminology remain unique to them. Secondary education typically takes place after six years of primary education and is followed by higher education, vocational education or employment. In most countries secondary education is compulsory, at least until the age of 16. Children typically enter the lower secondary phase around age 12. Compulsory education sometimes extends to age 20.