der Nachmittag
afternoon
The time of day between noon (around 12 p.m.) and evening (around 6 p.m.).
Am Nachmittag treffen wir uns im Café.
In the afternoon, we'll meet at the café.

afternoon
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afternoon
The time of day between noon (around 12 p.m.) and evening (around 6 p.m.).
Am Nachmittag treffen wir uns im Café.
In the afternoon, we'll meet at the café.

The word is built very logically: it consists of 'nach' (after) and 'der Mittag' (noon). So it literally refers to the time that comes after noon.
Ich habe erst nach dem Mittag Zeit für dich.
I only have time for you after noon.
To specify a time, you use 'am Nachmittag' (in the afternoon). When talking about today, yesterday, or tomorrow, you leave out the preposition: 'heute Nachmittag' (this afternoon), 'gestern Nachmittag' (yesterday afternoon), 'morgen Nachmittag' (tomorrow afternoon).
Am Nachmittag gehe ich meistens spazieren.
In the afternoon, I usually go for a walk.
The lowercase adverb 'nachmittags' means 'every afternoon' or 'usually in the afternoon'. It describes a regular action.
Das Geschäft hat nachmittags von 14 bis 18 Uhr geöffnet.
The shop is open in the afternoons from 2 to 6 p.m.
Common phrases are 'den Nachmittag mit etwas verbringen' (to spend the afternoon doing something), 'sich für den Nachmittag verabreden' (to arrange to meet in the afternoon), or the greeting 'Schönen Nachmittag!' (Have a nice afternoon!).
Wir haben den ganzen Nachmittag am See verbracht.
We spent the whole afternoon at the lake.
evening
'der Nachmittag' and 'der Abend' are both parts of the day and come directly one after the other. That is why learners can easily confuse them. 'der Nachmittag' begins after midday and lasts roughly until early evening. 'der Abend' begins later, when the day is coming to an end and it often gets dark. For appointments, the difference is important: 3 p.m. is afternoon, 8 p.m. is evening.

Heute Nachmittag gehen sie im Park spazieren.
This afternoon, they are going for a walk in the park.
Am Abend trifft Julian seine Freunde nach dem Kurs.
In the evening, Julian meets his friends after the course.