die Klausur
written exam
A written exam, usually taken at the end of a semester or course at school or university.
Nächste Woche schreibe ich eine wichtige Klausur in Mathematik.
Next week I'm taking an important written exam in mathematics.

exam / retreat
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written exam
A written exam, usually taken at the end of a semester or course at school or university.
Nächste Woche schreibe ich eine wichtige Klausur in Mathematik.
Next week I'm taking an important written exam in mathematics.

All meanings of 'Klausur' share one common idea: seclusion or being closed off. Whether it is the closed room for an exam, the closed meeting of a political party, or secluded life in a monastery or convent, it always involves separation from the outside world.
Während der Klausur durfte niemand den Raum verlassen.
During the exam, no one was allowed to leave the room.
In the context of exams, 'Klausur' (written exam) is often combined with the verbs 'schreiben' (to take/write), 'bestehen' (to pass), or 'verhauen' (colloquial for to fail). 'sich auf eine Klausur vorbereiten' (to prepare for a written exam) is also a fixed expression.
Ich muss das ganze Wochenende für meine Klausuren lernen.
I have to study for my exams all weekend.
The expression 'in Klausur gehen' (to go into retreat) is almost always used for political or business bodies. It means that a group withdraws in order to discuss matters intensively and without disturbance.
Der Vorstand ist für zwei Tage in Klausur gegangen, um die neue Strategie zu besprechen.
The board went into retreat for two days to discuss the new strategy.
The meaning 'Leben im Kloster' (life in a monastery or convent) is very specific and is used mainly in religious or historical texts. It is rare in everyday language.
Manche Mönchsorden leben in ewiger Klausur.
Some monastic orders live in perpetual seclusion.
test
'der Test' and 'die Klausur' can both be an exam at school, in a course, or at university. 'Test' is more general and can be short, oral, written, or even practical. 'Klausur' almost always means a written exam, often longer and more important, especially at school or university. They are confused because both can assess grades or performance. If you are talking about a larger written exam, 'Klausur' sounds more precise; for smaller or more general exams, 'Test' fits better.

Zur Vorbereitung auf die Klausur wiederholen wir heute die wichtigsten Themen.
To prepare for the written exam, we're reviewing the most important topics today.
Hat der Test vor dem Wochenende viel Zeit gekostet?
Did the test before the weekend take a lot of time?