Key Terms is derived from a Yahoo! Search capability we refer to internally as "Prisma."... Key Terms is an ordered terminological representation of what a document is about. The ordering of terms is based on each term's frequency and its positional and contextual heuristics...Each result contains up to 20 terms describing the document.
Add the parameter view=keyterms to the BOSS request to see the new functionality.
I wonder if this is at all related to the Key Term Extraction API that Yahoo! provides.