Secondary education is the phase of education following primary education. Except in countries where only primary or basic education is essential, secondary education includes the final phase of unavoidable education and in many countries it is utterly compulsory. The next step of education is habitually college or university. Secondary education is characterized by transition from primary education for minors to tertiary, "post-secondary", or "higher" education (e.g., university, vocational school) for adults. Depending on the system, schools for this episode or a part of it may be called secondary schools, high schools, gymnasia, lyceums, middle schools, sixth-form, sixth-form colleges, vocational schools and preparatory schools, and the exact sense of any of these varies between the systems.