der Zwanziger
twenty
A banknote worth twenty units of currency, especially a 20-euro note.
Kannst du mir einen Zwanziger für den Parkautomaten wechseln?
Can you change a twenty for me for the parking meter?

twenty / twenties
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twenty
A banknote worth twenty units of currency, especially a 20-euro note.
Kannst du mir einen Zwanziger für den Parkautomaten wechseln?
Can you change a twenty for me for the parking meter?

The word is derived from the number 'zwanzig' (twenty). This ending '-er' is often used in German to form nouns derived from numbers that refer to banknotes, decades, or age groups.
Gib mir bitte einen Fünfziger, ich habe nur einen Hunderter.
Please give me a fifty; I only have a hundred.
The article and the number (singular/plural) help you recognize the meaning. 'Der Zwanziger' in the singular usually refers to the banknote. 'Die Zwanziger' in the plural refers to the decade.
Hast du einen Zwanziger für den Kaffeeautomaten?
Do you have a twenty for the coffee machine?
In everyday life, 'der Zwanziger' (the twenty) is often used in situations where you want to pay or change money. A typical question is: 'Kannst du mir einen Zwanziger wechseln?' (Can you change a twenty for me?).
Ich habe dem Taxifahrer einen Zwanziger gegeben.
I gave the taxi driver a twenty.
The decade from 1920-1929 is often called 'die Goldenen Zwanziger' (the Golden Twenties). This expression refers to a time of economic upswing and cultural flourishing in many Western countries.
Der Film spielt im Berlin der Goldenen Zwanziger.
The film is set in the Berlin of the Golden Twenties.
To give a person's age roughly, people often use the phrases 'Anfang Zwanzig' (early twenties, about 20-23), 'Mitte Zwanzig' (mid-twenties, about 24-26), or 'Ende Zwanzig' (late twenties, about 27-29).
Sie hat ihr Studium mit Mitte Zwanzig abgeschlossen.
She finished her studies in her mid-twenties.
banknote
'der Zwanziger' and 'die Banknote' can both mean a paper bill. 'die Banknote' is the general and rather factual term for paper money. 'der Zwanziger' is much more specific: it means a banknote worth twenty, for example a 20-euro note. Learners may confuse the words because a 'Zwanziger' really is a banknote. But not every banknote is a 'Zwanziger'. If the exact value is important, you say 'Zwanziger'. If it is only about the type of money, 'Banknote' fits better.

An der Kasse gibt mir der Verkäufer einen Zwanziger zurück.
At the checkout, the cashier gives me a twenty back.
Diese alte Banknote wird im Museum gezeigt.
This old banknote is being displayed in the museum.