The Collaborative International Dictionary of English v.0.48:

avens \av"ens\ ([a^]v"[e^]nz), n.; pl. avens. [OF. avence.]
   (Bot.)
   A plant of the genus Geum, of the rose family, esp. Geum
   urbanum, or herb bennet. They may bear red, yellow, or white
   flowers.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

avens
    n 1: any of various perennials of the genus Geum having usually
         pinnate basal leaves and variously colored flowers