The Collaborative International Dictionary of English v.0.48:

Periwinkle \Per"i*win`kle\, n. [From AS. pinewincla a shellfish,
   in which pine- is fr. L. pina, pinna, a kind of mussel, akin
   to Gr. ?. Cf. Winkle.] (Zool.)
   Any small marine gastropod shell of the genus Littorina.
   The common European species (Littorina littorea), in Europe
   extensively used as food, has recently become naturalized
   abundantly on the American coast. See Littorina.
   [1913 Webster]

   Note: In America the name is often applied to several large
         univalves, as Fulgur carica, and Fulgur
         canaliculata.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Periwinkle \Per"i*win`kle\, n. [OE. pervenke, AS. pervince, fr.
   L. pervinca.] (Bot.)
   A trailing herb of the genus Vinca.
   [1913 Webster]

   Note: The common perwinkle (Vinca minor) has opposite
         evergreen leaves and solitary blue or white flowers in
         their axils. In America it is often miscalled myrtle.
         See under Myrtle.
         [1913 Webster]

WordNet (r) 3.0 (2006):

periwinkle
    n 1: chiefly trailing poisonous plants with blue flowers
    2: commonly cultivated Old World woody herb having large pinkish
       to red flowers [syn: periwinkle, rose periwinkle,
       Madagascar periwinkle, old maid, Cape periwinkle, red
       periwinkle, cayenne jasmine, Catharanthus roseus, Vinca
       rosea]
    3: small edible marine snail; steamed in wine or baked [syn:
       periwinkle, winkle]
    4: edible marine gastropod [syn: periwinkle, winkle]