der Mittag
noon
The middle of the day, the time around 12 o'clock.
Heute Mittag scheint die Sonne besonders stark.
The sun is especially strong at noon today.

noon / lunch
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noon
The middle of the day, the time around 12 o'clock.
Heute Mittag scheint die Sonne besonders stark.
The sun is especially strong at noon today.

The word 'Mittag' is made up of 'Mitte' (middle) and 'Tag' (day). So it literally describes the middle of the day.
Die Glocken läuten genau zur Mitte des Tages, also um 12 Uhr mittags.
The bells ring exactly in the middle of the day, that is, at 12 noon.
'Mittag' is a base word for many other useful time expressions. These include 'Vormittag' (morning/forenoon), 'Nachmittag' (afternoon), 'Mittagspause' (lunch break), and 'Mittagessen' (lunch).
Nach der Mittagspause fühle ich mich oft müde.
After the lunch break, I often feel tired.
To specify an exact time, 'Mittag' is often used with prepositions such as 'am Mittag' (at noon) or 'gegen Mittag' (around noon). Adverbs like 'heute' (today) or 'morgen' (tomorrow) can stand directly before it.
Wir treffen uns morgen Mittag vor dem Kino.
We're meeting tomorrow at noon in front of the cinema.
When 'Mittag' refers to the meal, it is almost always used in the fixed phrase 'zu Mittag'. Typical sentences are 'zu Mittag essen' (to eat lunch) or the question 'Was gibt es zu Mittag?' (What's for lunch?).
Ich esse heute nur einen Salat zu Mittag.
I'm only having a salad for lunch today.
afternoon
'der Nachmittag' and 'der Mittag' are both parts of the day, but they refer to different times. 'Mittag' is the time around 12 o'clock and can also mean the meal eaten at that time. 'Nachmittag' comes after that, so roughly between noon and evening. Learners may confuse the words because both are around the middle of the day. A practical rule is: at 12 o'clock you say 'Mittag'; after that, you say 'Nachmittag'.

Um Mittag mache ich eine Pause und esse etwas.
Around noon, I take a break and eat something.
Am Nachmittag hat meine Kollegin noch zwei Termine.
In the afternoon, my colleague still has two appointments.