der Diskus
discus
A heavy, lens-shaped disc used as a piece of sports equipment in athletics for discus throwing.
Der Athlet schleuderte den Diskus über 60 Meter weit.
The athlete hurled the discus over 60 meters.

discus / discus fish
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discus
A heavy, lens-shaped disc used as a piece of sports equipment in athletics for discus throwing.
Der Athlet schleuderte den Diskus über 60 Meter weit.
The athlete hurled the discus over 60 meters.

Both meanings come from the Latin word 'discus' (disc). Both the piece of sports equipment and the fish have a characteristic flat, round shape that resembles a disc.
Die Form des Fisches erinnert an eine flache Scheibe.
The shape of the fish is reminiscent of a flat disc.
The meaning can be easily inferred from the context. Verbs like 'werfen' (to throw) or 'schleudern' (to hurl) and words like 'Athletik' (athletics) or 'Stadion' (stadium) point to the sports equipment. Verbs like 'schwimmen' (to swim) or 'züchten' (to breed) and words like 'Aquarium' (aquarium) or 'Fisch' (fish) point to the ornamental fish.
Er trainiert jeden Tag das Werfen mit dem Diskus.
He practices throwing the discus every day.
In a sports context, 'Diskus' (discus) is mainly used with the verbs 'werfen' (to throw) and 'schleudern' (to hurl). The sport itself is called 'das Diskuswerfen' (discus throwing).
Beim Diskuswerfen muss man den Diskus so weit wie möglich schleudern.
In discus throwing, you have to hurl the discus as far as possible.
disc
'die Disc' and 'der Diskus' can both mean a round disc, especially in sports. Even so, they are not the same. 'Disc' fits a light plastic flying disc, for example in Ultimate Frisbee or disc golf. In sports, 'Diskus' means a heavy disc used for discus throwing in athletics. In addition, 'Diskus' can also mean a tropical fish. Learners may confuse the words because both have to do with a disc shape. As a practical rule: a light plastic disc in recreational or team sports is more likely 'Disc'; a heavy competition disc is 'Diskus'.

Der Sportler hebt den Diskus und bereitet sich auf den Wurf vor.
The athlete lifts the discus and prepares for the throw.
Beim Training wirft Lisa die Disc weit über das Feld.
During training, Lisa throws the disc far across the field.