affiliate; related party; contact
Refers to a person, company, or organization with which one has a business or professional relationship. Often used in corporate contexts to mean a party one deals with, such as a client, supplier, or partner.
Send year-end greetings to business partners and related parties.
Have a meeting with the person in charge at the related company.
関連会社 means an affiliated company, often in a corporate group. 関係先 is more general and can refer to any related party, not necessarily a company.
Compound of 関係 (relation) and 先 (destination, party). The exact historical derivation is uncertain, but the word follows a common pattern for business-related terms.