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):

Vinca minor
    n 1: widely cultivated as a groundcover for its dark green shiny
         leaves and usually blue-violet flowers [syn: myrtle,
         Vinca minor]