der Knopf
button
A small, usually round part on clothing that is used for fastening by being pushed through a buttonhole.
Mir ist ein Knopf vom Hemd abgefallen.
A button fell off my shirt.

button
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button
A small, usually round part on clothing that is used for fastening by being pushed through a buttonhole.
Mir ist ein Knopf vom Hemd abgefallen.
A button fell off my shirt.

The expression 'auf Knopfdruck' (at the push of a button) is very common. It means that something works immediately, automatically, or very easily.
Die Kaffeemaschine macht auf Knopfdruck einen perfekten Espresso.
The coffee machine makes a perfect espresso at the push of a button.
The verb reveals the meaning: a button on a device is normally 'gedrückt' (pressed). A button on clothing is 'angenäht' (sewn on), 'zugeknöpft' (buttoned up), or it 'fällt ab' (falls off).
Du musst den roten Knopf drücken, um die Maschine zu stoppen.
You have to press the red button to stop the machine.
In connection with clothing, the verbs 'annähen' (to sew on), 'abreißen' (to tear off), or 'abfallen' (to fall off) are typical. The corresponding actions are called 'zuknöpfen' (to button up/close) and 'aufknöpfen' (to unbutton/open).
Er knöpfte sein Hemd bis ganz nach oben zu.
He buttoned his shirt all the way to the top.
For devices, people often talk about the 'Startknopf' (start button), 'Notknopf' (emergency button), or 'roter Knopf' (red button). The typical actions are 'einen Knopf drücken' (to press a button) or sometimes also 'einen Knopf drehen' (to turn a knob).
Im Notfall drücken Sie bitte den großen roten Knopf.
In an emergency, please press the large red button.
This meaning for the clitoris is very colloquial and casual. This expression is not used in formal, medical, or scientific contexts.
In dem Aufklärungsbuch für Jugendliche wurde auch der Begriff „Knopf“ erklärt.
The sex education book for teenagers also explained the term 'Knopf' (button).
button
'Knopf' and 'Button' can both mean something small that you use or see on clothing. 'Knopf' is the normal German word for a part on clothing or for a physical button on a device. 'Button' is used mainly for a button on a screen, for example in an app or on a website. A 'Button' can also be a small pin with a picture or slogan. There is a risk of confusion because the English word 'button' often means 'Knopf'. In German, however, you almost always say 'Knopf' for clothing and often 'Button' for apps.

An meiner Jacke ist heute Morgen ein Knopf abgegangen.
A button came off my jacket this morning.
Auf der Webseite findest du den Button zum Bezahlen unten rechts.
On the website, you can find the button for paying at the bottom right.