der Stier
bull
An adult male bovine animal.
Der Bauer führte den kräftigen Stier auf die Weide.
The farmer led the powerful bull out to the pasture.

bull / Taurus
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bull
An adult male bovine animal.
Der Bauer führte den kräftigen Stier auf die Weide.
The farmer led the powerful bull out to the pasture.

You can tell from the context whether the animal or the zodiac sign is meant. Words like 'Horoskop' (horoscope), 'Sternzeichen' (zodiac sign), or 'geboren sein' (to be born) clearly point to the astrological meaning. In agricultural or biological contexts, the animal is meant.
Auf dem Bauernhof nebenan gibt es Kühe und einen großen Stier.
On the farm next door, there are cows and a large bull.
'Stier' (bull) is the base word for other terms related to the animal. A well-known example is 'Stierkampf' (bullfight), the fight between a person and a bull.
In Spanien sind Stierkämpfe eine umstrittene Tradition.
In Spain, bullfighting is a controversial tradition.
The idiom 'den Stier bei den Hörnern packen' (to take the bull by the horns) is a fixed unit. It is used to express that someone accepts a challenge bravely and directly instead of hesitating.
Statt uns weiter zu beschweren, sollten wir den Stier bei den Hörnern packen und eine Lösung finden.
Instead of continuing to complain, we should take the bull by the horns and find a solution.
cow
'Kuh' and 'Stier' both refer to adult bovine animals, but they are opposites in terms of sex. A 'Kuh' is female; a 'Stier' is male. Learners may confuse the words because the animals look similar and often appear in the same context, for example on a farm. For the animal that gives milk, you say 'Kuh'. For an adult male bovine, you say 'Stier'.

Der Stier stand allein auf der Weide und wirkte sehr kräftig.
The bull stood alone in the pasture and looked very powerful.
Das Kind gab der Kuh im Stall vorsichtig etwas Heu.
The child carefully gave the cow some hay in the stable.