Related party
A person or company with close ties to the reporting company, such as an investee it partly owns, a big owner, or a director. Deals between them may not happen on normal market terms, so the 10-K has to disclose them: what was bought and sold, and what is still owed.
Simple example
A software company owns 30% of a startup, and the startup buys $50 of cloud services from it. The software company must report that $50 as related-party revenue, plus anything the startup still owes it.