der Gesellschafter
partner
A person or company that owns shares in a company, such as a GmbH or KG, and is therefore a co-owner.
Die beiden Gesellschafter haben beschlossen, die Firma zu verkaufen.
The two partners decided to sell the company.

partner / companion
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partner
A person or company that owns shares in a company, such as a GmbH or KG, and is therefore a co-owner.
Die beiden Gesellschafter haben beschlossen, die Firma zu verkaufen.
The two partners decided to sell the company.

The word is derived from 'Gesellschaft' (company/society). This word can refer both to a company in a business sense and to social togetherness, which explains the two meanings of 'Gesellschafter' (partner/companion): a partner in a company or a social companion.
Er ist einer von drei Gesellschaftern der neuen IT-Firma.
He is one of three partners in the new IT company.
You can recognize the meaning from the context. If it is about companies, shares, or contracts, it means the co-owner. If it is about travel, accompaniment, or social occasions, it means the companion.
Die Gesellschafter treffen sich einmal im Jahr zur Versammlung.
The partners meet once a year for the assembly.
In a business context, there are fixed terms like 'geschäftsführender Gesellschafter' (managing partner, actively runs the company) or 'stiller Gesellschafter' (silent partner, only financially involved). The legal form is also often mentioned, e.g. 'Gesellschafter einer GmbH' (partner/shareholder of a GmbH).
Als stiller Gesellschafter hat er kein Mitspracherecht im Tagesgeschäft.
As a silent partner, he has no say in day-to-day business.
The meaning 'Begleitperson' (companion) is rather rare today and often sounds old-fashioned or very formal. You find it more often in historical novels or films than in modern everyday language.
In dem Roman sucht ein älterer Herr einen Gesellschafter für seine Spaziergänge.
In the novel, an elderly gentleman is looking for a companion for his walks.
managing director
'der Geschäftsführer' and 'der Gesellschafter' both belong to the world of companies, especially in the context of a GmbH. They are easy to confuse because they sound similar and can both be important roles in a company. 'der Geschäftsführer' runs the company day to day and makes decisions for the ongoing business operations. 'der Gesellschafter' owns shares in the company and is therefore a co-owner. A partner does not have to run the company personally. Conversely, a managing director can also be an employee and own no shares.

Sofia ist Gesellschafterin der kleinen Firma und besitzt zehn Prozent der Anteile.
Sofia is a partner in the small company and owns ten percent of the shares.
Bei der Bewerbung hat der Geschäftsführer persönlich mit den Kandidaten gesprochen.
During the application process, the managing director spoke personally with the candidates.