der Einwurf
interjection
A brief remark or comment that is brought into a conversation or discussion.
Sein kritischer Einwurf brachte die Diskussion auf einen wichtigen Punkt.
His critical interjection brought the discussion to an important point.

interjection / throw-in / slot
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interjection
A brief remark or comment that is brought into a conversation or discussion.
Sein kritischer Einwurf brachte die Diskussion auf einen wichtigen Punkt.
His critical interjection brought the discussion to an important point.

'Der Einwurf' comes from the verb 'einwerfen' (to throw in). This literal meaning connects all the senses: you 'throw' a comment into a discussion, a ball onto the field, or a letter into the box.
Darf ich kurz etwas einwerfen?
May I briefly add something?
When an 'Einwurf' is a remark, it is often combined with the verb 'machen' (to make). Typical adjectives are 'kurz' (brief), 'wichtig' (important), or 'kritisch' (critical).
Danke für diesen wichtigen Einwurf.
Thank you for this important interjection.
In a sports context, the phrase 'Einwurf für...' (throw-in for...) is often used to indicate which team may throw the ball in. The situation itself is described with 'es gibt Einwurf' (there is a throw-in).
Nach dem Foul gab es Einwurf für die gegnerische Mannschaft.
After the foul, there was a throw-in for the opposing team.
As an opening or slot, 'Einwurf' is often the second part of a compound noun. Examples are 'der Briefeinwurf' (mail slot) for letters or 'der Münzeinwurf' (coin slot) for coins.
Der Briefeinwurf an unserer Haustür ist zu klein für große Umschläge.
The mail slot on our front door is too small for large envelopes.
comment
'der Kommentar' and 'der Einwurf' can both be a remark about a topic. 'Kommentar' is broader and can be spoken or written, for example on the internet, in a newspaper, or in a conversation. 'Einwurf' usually means a short remark that someone adds during a conversation or discussion. The words can be confused because both can be a reaction to something. If the remark is more detailed or written, 'Kommentar' fits better. If someone briefly speaks up in an ongoing discussion, 'Einwurf' is more precise.

Mit einem kurzen Einwurf erinnert sie alle an den nächsten Termin.
With a brief interjection, she reminds everyone of the next appointment.
Die Gruppe hat unter dem Foto einen freundlichen Kommentar geschrieben.
The group wrote a friendly comment under the photo.