Chocolate, as defined by the FDA, that contains at least 35% cocoa solids.  Most bittersweet chocolate bars have at least 50% chocolate liquor, with some bars pushing 70-80% chocolate liquor.  Bittersweet  chocolate usually has a deeper, more bitter taste than sweet, dark, or semi-sweet bars.  However, the total number of sugar in the chocolate is not regulated, so one manufacturer’s bittersweet bar can taste sweeter than another’s semi-sweet bar.