If you open a lemonade stand with your brother, then your brother is your business partner.
Partner means "to pair up." At a square dance, the call "partner up!" means it's time to find someone to dance with. In business, there are often multiple partners — each one owns a share of the business, and has a share of the risk. Some people call their spouse or the person they're in a long-term relationship with their partner — it has more egalitarian senses than "husband" or "wife," and signals commitment even if you're not married.