From Yahoo! Search Blog -
"Yahoo! Search is taking another step in extending the Yahoo! Open Strategy with the launch of Yahoo! Search BOSS, a web services platform that allows developers and companies to create and launch web-scale search products by utilizing the same infrastructure and technology that powers Yahoo! Search."
BOSS means Build your Own Search Service
Here is the BOSS logo -
Get more information at http://www.ysearchblog.com/archives/000599.html and http://developer.yahoo.com/search/boss/