A partner can have more than one meaning it can mean.
1. When two or more people set up a business together they are each partners and there would be legal definitions also; that may be a little different between the various laws of each English speaking country.
2 You can be or became a partner in an already established business or corporation such as a lawyer firm, or a dentist firm or a research firm etc, the list is long. Your name would appear on the companies/firms letterheads and possibly also on the legal documents of the firm in the public records hall of that city or country. Once again the laws and rules would vary between each English speaking country.