Masthead is the brand name given by YouTube to his billboard ad unit offered on his homepage. The masthead is a 970x250 ads that occupy the top third to half of the home page.
The YouTube’s masthead is sold on a day per day basis at a flat fee. For 2011 YouTube charged $375,000 per day for a home page "masthead" ad.
Masthead costs for different countries in 2011: