der Schiit
Shiite
A follower of Shiism, one of the two main branches of Islam.
Der Nachbar meiner Tante ist ein gläubiger Schiit aus dem Irak.
My aunt's neighbor is a devout Shiite from Iraq.

Shiite
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Shiite
A follower of Shiism, one of the two main branches of Islam.
Der Nachbar meiner Tante ist ein gläubiger Schiit aus dem Irak.
My aunt's neighbor is a devout Shiite from Iraq.

This noun belongs to the N-declension. That means it takes the ending '-en' in all cases except the nominative singular. Learners often make this mistake.
Ich habe gestern mit einem Schiiten gesprochen.
I spoke with a Shiite yesterday.
The word 'der Schiit' (Shiite) belongs to a word family. The religious branch is called 'der Schiismus' (Shiism), and the related adjective is 'schiitisch' (Shiite/Shi'a).
Er kennt sich gut mit dem Schiismus aus.
He knows a lot about Shiism.
This word is often used with geographical information or descriptive adjectives to make someone's origin or religious practice clear.
Die meisten Schiiten leben im Iran und im Irak.
Most Shiites live in Iran and Iraq.
Muslim
'der Muslim' and 'der Schiit' both refer to Islam. 'Muslim' is the general term for a male follower of Islam. 'Schiit' is more specific and means a follower of Shiism, a particular branch within Islam. The words can be confused because a Shiite is also a Muslim. But not every Muslim is a Shiite. If you only mean the religion in general, 'Muslim' fits. If the specific branch is important, you say 'Schiit'.

Im Unterricht erklärt die Lehrerin, dass ein Schiit zu einer Richtung des Islam gehört.
In class, the teacher explains that a Shiite belongs to a branch of Islam.
Ali ist Muslim und feiert mit seiner Familie das Zuckerfest.
Ali is Muslim and celebrates Eid al-Fitr with his family.