This is the 3005th most frequent Arabic word.
أميركي
American.
Here, "أميركية" is used as an adjective to describe the noun "culture" (الثقافة) as being American.
أنا أحب الثقافة الأميركية.
I love American culture.
In this case, "أميركي" describes the nationality of the person, used here as an adjective.
التقيت بشخص أميركي في المؤتمر.
I met an American person at the conference.
Here, "الأميركي" is used in a construct phrase (إضافة) to associate the noun "football team" (منتخب كرة القدم) with "American."