
das Kissen
pillow
A bag filled with soft material that is used in bed as support for the head while sleeping.
Ich kann ohne mein spezielles Nackenkissen nicht gut schlafen.
I can't sleep well without my special neck pillow.
pillow / cushion
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pillow
A bag filled with soft material that is used in bed as support for the head while sleeping.
Ich kann ohne mein spezielles Nackenkissen nicht gut schlafen.
I can't sleep well without my special neck pillow.

cushion
A soft pad used for sitting, kneeling, or decoration on furniture such as sofas or chairs.
Für mehr Gemütlichkeit legte sie bunte Kissen auf das Sofa.
To make it cozier, she put colorful cushions on the sofa.
Whether a 'Kissen' (pillow / cushion) is used for sleeping ('Kopfkissen', pillow) or for decoration ('Sofakissen', sofa cushion) can be recognized from context. Words like 'Bett' (bed) and 'schlafen' (to sleep) point to the first meaning, while 'Sofa' (sofa), 'Stuhl' (chair), or 'Dekoration' (decoration) point to the second.
Er schläft am besten auf einem sehr flachen Kissen.
He sleeps best on a very flat pillow.
To be more precise, compound words are often used. A 'Kopfkissen' (pillow) is always for sleeping, while a 'Sofakissen' (sofa cushion), 'Sitzkissen' (seat cushion), or 'Zierkissen' (decorative cushion) lies on furniture.
Für die Gartenstühle brauchen wir noch bequeme Sitzkissen.
We still need comfortable seat cushions for the garden chairs.
In Austria, the word 'der Polster' is used very frequently for 'Kissen' (pillow / cushion). This word can refer both to a pillow for sleeping and to a decorative cushion.
Kannst du bitte den Polster aufschütteln?
Could you please fluff up the pillow?
edition / condition
Auflage and Kissen can both serve as a support. In its technical sense, Auflage is the surface on which a component rests, whereas Kissen is a soft, filled cushion for furniture. Auflage is appropriate for a component, while Kissen is used for comfortable sitting.

Für den harten Stuhl legte sie ein weiches Kissen auf die Sitzfläche.
She put a soft cushion on the seat of the hard chair.
Das Bauteil ruht sicher auf der Auflage.
The component rests securely on the support surface.