The term business partner ideally hints to term an entity which has some kind of interest in the business. It is usually endowed with some particular job roles and function in aid of performing the business process.
Source: https://www.apprisia.com/blog/sap-ecc/improve-your-business-efficiency-with-sap-business-partner/
Source: https://www.apprisia.com/blog/sap-ecc/improve-your-business-efficiency-with-sap-business-partner/