Grapefruit vs. Pampelmuse
While both Grapefruit and Pampelmuse can be used to refer to the grapefruit, they have distinct primary meanings and usage, which can lead to confusion. Grapefruit is the standard, unambiguous term, whereas Pampelmuse has a different botanical meaning but is also used colloquially for grapefruit.
Grapefruit
A2Grapefruit is the modern, standard, and universally understood German word for the hybrid citrus fruit known as grapefruit (Citrus × paradisi). It is the term you will almost always find in supermarkets, recipes, and everyday conversation.
Zum Frühstück esse ich gerne eine halbe Grapefruit mit Zucker.
(For breakfast, I like to eat half a grapefruit with sugar.)
Dieser Saft ist aus rosa Grapefruits gemacht und schmeckt leicht bitter.
(This juice is made from pink grapefruits and tastes slightly bitter.)
Kannst du im Supermarkt bitte Grapefruits statt Orangen kaufen?
(At the supermarket, can you please buy grapefruits instead of oranges?)
Für diesen Cocktail benötigst du den frischen Saft einer Grapefruit.
(For this cocktail, you need the fresh juice of a grapefruit.)
Pampelmuse
B2Botanically, Pampelmuse is the correct German term for the pomelo (Citrus maxima), a separate, much larger citrus fruit from which the grapefruit is descended. However, Pampelmuse is also used colloquially and regionally (especially in Northern Germany) as a synonym for grapefruit, often by older generations.
Eine echte Pampelmuse hat eine sehr dicke Schale und ist oft süßer als eine Grapefruit.
(A real pomelo has a very thick peel and is often sweeter than a grapefruit.)
Meine Großmutter nennt Grapefruits immer noch Pampelmusen, so wie sie es gelernt hat.
(My grandmother still calls grapefruits Pampelmusen, just as she learned it.)
In Hamburg habe ich jemanden nach dem Weg zum Obststand für Pampelmusen gefragt.
(In Hamburg, I asked someone for directions to the fruit stand for grapefruits.)
Der botanische Unterschied zwischen einer Pampelmuse und einer Grapefruit ist beträchtlich.
(The botanical difference between a pomelo and a grapefruit is considerable.)
Summary
The main difference is clarity and common usage. Use Grapefruit to refer to a grapefruit to be universally understood. Pampelmuse correctly refers to the pomelo, but its colloquial use for grapefruit in certain regions and by older speakers makes it ambiguous. To be precise, it's best to use Grapefruit for grapefruits and Pomelo (which is also common in German) for pomelos, thereby avoiding the Pampelmuse confusion altogether.