der Amerikaner
American
A male resident or citizen of the United States of America (USA).
Mein neuer Kollege ist Amerikaner und kommt aus Texas.
My new colleague is American and comes from Texas.

American
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American
A male resident or citizen of the United States of America (USA).
Mein neuer Kollege ist Amerikaner und kommt aus Texas.
My new colleague is American and comes from Texas.

Whether a person or a pastry is meant is always revealed by the context. Verbs like 'essen' (to eat), 'kaufen' (to buy), or places like 'Bäckerei' (bakery) point to the pastry. Verbs like 'kennenlernen' (to meet), 'sprechen' (to speak), or 'sein' (to be) point to the person.
Ich habe heute einen netten Amerikaner kennengelernt.
I met a nice American today.
The word is formed from the country name 'Amerika' (America) and the ending '-aner', which is often used for names of inhabitants. The corresponding adjective is 'amerikanisch' (American).
Die amerikanische Kultur ist weltweit bekannt.
American culture is known worldwide.
In general German usage, 'der Amerikaner' (the American) almost exclusively refers to a person from the USA. For people from other countries on the American continent, more precise terms like 'Kanadier' (Canadian) or 'Mexikaner' (Mexican) are used.
Er ist Amerikaner, aber seine Eltern stammen aus Deutschland.
He is American, but his parents come from Germany.
American
'der Amerikaner' and 'der Ami' can both mean a man or person from the USA. 'der Amerikaner' is neutral and fits in almost all situations, including official or polite conversations. 'der Ami' is colloquial and sounds more casual. Depending on the tone, it can sound friendly, but also somewhat derogatory. At school, at work, or in formal texts, 'Amerikaner' is usually safer. 'Ami' is more suitable for casual conversations among friends.

Der Amerikaner arbeitet seit zwei Jahren in Berlin.
The American has been working in Berlin for two years.
Mein Bruder nennt seinen Freund aus Chicago manchmal scherzhaft einen Ami.
My brother sometimes jokingly calls his friend from Chicago an Ami.