der Onkel
uncle
The brother of one's father or mother.
Mein Onkel mütterlicherseits ist Arzt.
My uncle on my mother's side is a doctor.

uncle
Loading your saved state.
uncle
The brother of one's father or mother.
Mein Onkel mütterlicherseits ist Arzt.
My uncle on my mother's side is a doctor.

To make clear whether he is the brother of the father or the mother, you can add the adverbs 'väterlicherseits' (on the father's side) or 'mütterlicherseits' (on the mother's side).
Mein Onkel väterlicherseits lebt in Hamburg.
My uncle on my father's side lives in Hamburg.
If the uncle is not the brother of a parent but the husband of an aunt, he can be referred to as an 'angeheirateter Onkel' (uncle by marriage). In everyday life, however, this distinction is rarely made.
Streng genommen ist er nur mein angeheirateter Onkel, aber ich sehe ihn als Teil der Familie.
Strictly speaking, he is only my uncle by marriage, but I see him as part of the family.
The form of address 'Onkel' (uncle) for a friend of the family is very colloquial and is used mainly by children or when speaking with children. It expresses a close, family-like relationship.
Der Onkel Klaus kommt heute Abend zu Besuch.
Uncle Klaus is coming to visit this evening.
gentleman / Mr.
'der Herr' and 'der Onkel' can both refer to an adult man, but the context is very different. 'Herr' is used formally or politely, especially with a last name or for an unknown man. 'Onkel' normally refers to a man in the family: the brother of one's mother or father, or the husband of one's aunt. Children sometimes also use 'Onkel' in a friendly way for a familiar adult man. Learners may confuse the words when the meaning is simply 'a man'. For unfamiliar or formal situations, 'Herr' fits; for family or familiar children's language, 'Onkel' fits.

Am Samstag besucht mein Onkel uns mit seiner Familie.
On Saturday, my uncle is visiting us with his family.
Frau Meier begrüßt den Herrn am Empfang sehr freundlich.
Mrs. Meier greets the gentleman at reception very warmly.