der Mitbegründer
co-founder
A person who, together with others, creates or establishes a company, organization, movement, or academic discipline.
Steve Wozniak ist der Mitbegründer von Apple.
Steve Wozniak is the co-founder of Apple.

co-founder
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co-founder
A person who, together with others, creates or establishes a company, organization, movement, or academic discipline.
Steve Wozniak ist der Mitbegründer von Apple.
Steve Wozniak is the co-founder of Apple.

The word consists of the prefix 'Mit-' and the noun 'Begründer' (founder). The prefix 'Mit-' always means 'together with others'. A 'Mitbegründer' (co-founder) is therefore someone who founds an organization not alone, but as part of a team.
Er war nicht der alleinige Chef, sondern einer der Mitbegründer des Start-ups.
He was not the sole boss, but one of the co-founders of the start-up.
To name the company or organization that was founded, German typically uses the preposition 'von' (of). The structure is: '(jemand) ist Mitbegründer von [Name der Firma]' ((someone) is co-founder of [company name]).
Bill Gates ist der Mitbegründer von Microsoft.
Bill Gates is the co-founder of Microsoft.
The word is often used in connection with organizations or ideas. Typical combinations are 'Mitbegründer einer Firma' (co-founder of a company), 'eines Vereins' (of an association/club), 'einer Partei' (of a party), or 'einer Bewegung' (of a movement).
Sie war eine Mitbegründerin der lokalen Umweltschutzbewegung.
She was a co-founder of the local environmental protection movement.
founder
'der Gründer' and 'der Mitbegründer' both mean a person who brings something new into existence, for example a company or organization. The most important difference is the number of people involved. 'Gründer' can generally mean a founding person and often also sounds appropriate when one person is especially in focus. 'Mitbegründer' says more precisely that this person founded something together with others. The words can be confused because a co-founder is also a founder. But if you want to emphasize that there were several founders, 'Mitbegründer' is more precise.

Ein Mitbegründer der Organisation hat den ersten Plan zusammen mit zwei Freunden geschrieben.
A co-founder of the organization wrote the first plan together with two friends.
Der Gründer des kleinen Unternehmens arbeitet noch jeden Tag im Laden mit.
The founder of the small company still works in the shop every day.