Yahoo! expanded the functionality of their BOSS API with a new BOSS features called Key Terms, and is derived from a Yahoo! Search technology that powers Search Assist, which provides search suggestions and enables searchers to explore concepts related to the query.
The Key Terms feature uses term frequency and positional and contextual heuristics to return ordered lists that describe a web page. Each result returned for a query includes associated metadata of up to 20 terms that describe that result.
More info: Yahoo! Search Blog