Bittersweet

both bitter and sweet to the taste:
bittersweet chocolate.
both pleasant and painful or regretful:
a bittersweet memory.
Also called woody nightshade. a climbing or trailing plant, Solanum dulcamara, of the nightshade family, having small, violet, star-shaped flowers with a protruding yellow center and scarlet berries.
Also called climbing bittersweet. any climbing plant of the genus Celastrus, bearing orange capsules opening to expose red-coated seeds, especially C. scandens.
pleasure mingled with pain or regret:
the bittersweet of parting.
Contemporary Examples

Paul Auster, Bernhard Schlink, and More of This Week’s Hot Reads: Aug. 13, 2012 Mythili Rao August 11, 2012
An American Prairie Feast Sophie Menin July 16, 2010
The Real Housewives of New York City’s Alex McCord Lets Loose Nicole LaPorte May 24, 2011
Thank You Iron Dome Orly Halpern March 10, 2012
Mother and Son, Wars and Recipes Matt Beynon Rees May 21, 2009

Historical Examples

Tess of the Storm Country Grace Miller White
The Child’s Rainy Day Book Mary White
La Sorcire: The Witch of the Middle Ages Jules Michelet
The Children’s Book of Birds Olive Thorne Miller
Freckles Gene Stratton-Porter

noun
any of several North American woody climbing plants of the genus Celastrus, esp C. scandens, having orange capsules that open to expose scarlet-coated seeds: family Celastraceae
another name for woody nightshade
adjective
tasting of or being a mixture of bitterness and sweetness
pleasant but tinged with sadness