der Sonderpreis
special price
A temporarily reduced price for a product or service; a special offer.
Das Notebook gibt es diese Woche zum Sonderpreis für nur 499 Euro.
The laptop is available this week at a special price of only 499 euros.

special price / special prize
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special price
A temporarily reduced price for a product or service; a special offer.
Das Notebook gibt es diese Woche zum Sonderpreis für nur 499 Euro.
The laptop is available this week at a special price of only 499 euros.

The word is made up of 'Sonder-' (which means 'special') and 'Preis' (price/prize). So a 'Sonderpreis' is always a 'special price/prize', either as a discount or as an award.
Für ihre herausragende Leistung erhielt sie einen Sonderpreis.
She received a special prize for her outstanding performance.
You can recognize the meaning from the context. If it is about shopping, offers, and money, it means the discount. If it is about competitions, juries, and awards, it means the honorary prize.
Diese Kaffeemaschine gibt es nur heute zum Sonderpreis.
This coffee machine is available at a special price today only.
When the reduced price is meant, the preposition 'zu' is almost always used: 'etwas zum Sonderpreis kaufen' (to buy something at a special price) or 'etwas zum Sonderpreis anbieten' (to offer something at a special price).
Ich habe die Jacke zum Sonderpreis bekommen.
I got the jacket at a special price.
In the context of an award, a 'Sonderpreis' is often 'verliehen' (awarded), 'vergeben' (given), 'erhalten' (received), or 'gewonnen' (won).
Dem jungen Musiker wurde der Sonderpreis der Jury verliehen.
The young musician was awarded the jury's special prize.
discount
'der Rabatt' and 'der Sonderpreis' both have to do with a lower price. 'Rabatt' emphasizes the deduction from the original price, often as a percentage or a specific amount. 'Sonderpreis' is the new special price itself, usually for a short time or as an offer. They can be confused because both show, when shopping, that something is cheaper. More precisely: The 'Rabatt' explains how much is deducted; the 'Sonderpreis' is the price you pay in the end.

Diese Lampe kostet nur heute einen Sonderpreis von 29 Euro.
This lamp is available today only at a special price of 29 euros.
Für das Werkzeug bekommen die Eltern beim Einkaufen einen Rabatt.
The parents get a discount on the tool while shopping.