die Kleinigkeit
trifle
A thing that is small, unimportant, or of little value. It can refer to a small problem, a small gift, or a small amount.
Mach dir keine Sorgen, das ist nur eine Kleinigkeit.
Don't worry, it's only a trifle.

trifle / snack
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trifle
A thing that is small, unimportant, or of little value. It can refer to a small problem, a small gift, or a small amount.
Mach dir keine Sorgen, das ist nur eine Kleinigkeit.
Don't worry, it's only a trifle.

'Kleinigkeit' is derived from the adjective 'klein' (small). The ending '-keit' turns adjectives into nouns that describe a quality or thing. These nouns are always feminine.
Die Sauberkeit des Zimmers war eine Selbstverständlichkeit.
The cleanliness of the room was a matter of course.
In social situations, 'Kleinigkeit' (trifle, little something) is often used to sound modest. You say it when giving someone a gift ('Das ist nur eine Kleinigkeit' — It's only a little something) or when someone thanks you for help ('Das war doch eine Kleinigkeit' — It was nothing really).
Danke für das tolle Geschenk! – Ach, das ist doch nur eine Kleinigkeit.
Thank you for the wonderful gift! – Oh, it's only a little something.
Common phrases are 'sich über Kleinigkeiten aufregen' (to get upset about trifles) or 'sich um Kleinigkeiten streiten' (to argue about trifles) when referring to unimportant problems.
Bitte streitet euch nicht wegen so einer Kleinigkeit.
Please don't argue over such a trifle.
You can almost always recognize the meaning 'small snack' from the verb 'essen' (to eat). The fixed phrase is 'eine Kleinigkeit essen' (to have a little something to eat).
Ich habe Hunger, lass uns schnell eine Kleinigkeit essen gehen.
I'm hungry; let's quickly go have a small snack.
detail
'das Detail' and 'die Kleinigkeit' can both refer to something small. The difference lies in the evaluation. A 'Detail' is a small part of a larger whole and can be very important. A 'Kleinigkeit' is more like something small, unimportant, or of little value. Learners may confuse the words because both have to do with 'klein' (small). If a small point is important for accuracy, planning, or understanding, 'Detail' fits better. If you want to say that something is not so important or is only a small matter, use 'Kleinigkeit'.

Leon hat sich wegen einer Kleinigkeit entschuldigt.
Leon apologized for a small thing.
Beim Vertrag sollten wir jedes Detail genau prüfen.
We should check every detail of the contract carefully.