1. [n]  twining shrub of North America having three-valved yellow capsules enclosing scarlet seeds
2. [n]  poisonous perennial Old World vine having violet flowers and oval coral-red berries; widespread weed in North America
3. [adj]  tinged with sadness; "a movie with a bittersweet ending"
4. [adj]  having a taste that is a mixture of bitterness and sweetness

BITTERSWEET is a 11 letter word that starts with B.

1. \Bit"ter*sweet`\, a.
   Sweet and then bitter or bitter and then sweet; esp. sweet
   with a bitter after taste; hence (Fig.), pleasant but
   painful.
   
   

2. \Bit"ter*sweet`\, n.
   1. Anything which is bittersweet.
   
   2. A kind of apple so called. --Gower.
   
   3. (Bot.)
      (a) A climbing shrub, with oval coral-red berries
          ({Solanum dulcamara}); woody nightshade. The whole
          plant is poisonous, and has a taste at first sweetish
          and then bitter. The branches are the officinal
          dulcamara.
      (b) An American woody climber ({Celastrus scandens}),
          whose yellow capsules open late in autumn, and
          disclose the red aril which covers the seeds; -- also
          called {Roxbury waxwork}.