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.

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
'die Auflage' and 'das Kissen' can both refer to something on which something lies or rests. The difference is in the context. In engineering or construction, 'die Auflage' is the surface or place on which a component rests. 'das Kissen' is a soft pillow or cushion for the head, for sitting, kneeling, or as decoration. There is a risk of confusion because both can have to do with a support or base. In everyday life, however, you say 'Kissen' when it is soft and intended for people. 'Auflage' sounds more technical and describes the function of supporting something that rests on it.

Vor dem Termin legte er sich kurz ein Kissen in den Rücken.
Before the appointment, he briefly put a cushion behind his back.
Die Auflage muss eben sein, damit das schwere Bauteil sicher liegt.
The support surface must be level so that the heavy component rests securely.