die Verkürzung
shortening
The act of reducing something in its length, duration, or scope.
Die Verkürzung der Arbeitszeit ist ein zentrales Thema der Gewerkschaft.
The shortening of working hours is a central issue for the union.

shortening / abbreviation
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shortening
The act of reducing something in its length, duration, or scope.
Die Verkürzung der Arbeitszeit ist ein zentrales Thema der Gewerkschaft.
The shortening of working hours is a central issue for the union.

The noun 'Verkürzung' (shortening; abbreviation) belongs to the word family of 'kurz' (short). From the adjective 'kurz' comes the verb 'verkürzen' (to shorten; to make something shorter). 'Die Verkürzung' is then the action or the result of this process.
Der Autor musste den Text für die Veröffentlichung verkürzen.
The author had to shorten the text for publication.
To express what is being shortened, the genitive is used very frequently. This structure is typical of written language.
Die Verkürzung des Schuljahres wurde intensiv diskutiert.
The shortening of the school year was discussed intensively.
In the meaning of 'etwas kürzer machen' (to make something shorter), the word is often used in connection with time, deadlines, or routes. Typical combinations are 'Verkürzung der Wartezeit' (shortening of the waiting time), 'Verkürzung der Frist' (shortening of the deadline), or 'Verkürzung des Verfahrens' (shortening of the procedure).
Durch den neuen Tunnel ergibt sich eine erhebliche Verkürzung der Fahrzeit.
The new tunnel results in a significant shortening of the travel time.
You can recognize the medical meaning by the fact that 'Verkürzung' (shortening) is used together with body parts such as 'Muskeln' (muscles), 'Sehnen' (tendons), or 'Bändern' (ligaments), or in a health context.
Eine Verkürzung der Wadenmuskulatur kann zu Schmerzen im Fuß führen.
Shortening of the calf muscles can lead to pain in the foot.
extension / extra time
'die Verlängerung' and 'die Verkürzung' form a clear contrast. With a 'Verlängerung' (extension), something becomes longer, for example a deadline, a contract, or an object. With a 'Verkürzung' (shortening), something becomes shorter, for example a period of time, a text, or a route. So the difference is the direction of the change: more length or more time with 'Verlängerung', less length or less time with 'Verkürzung'.

Beim Training hat die Verkürzung der Pause viele Spieler müde gemacht.
During training, shortening the break made many players tired.
Yusuf freut sich über die Verlängerung seines Mietvertrags.
Yusuf is happy about the extension of his rental contract.