The Collaborative International Dictionary of English v.0.48:

Anthesis \An*the"sis\, n. [Gr. ? bloom, fr. 'anqei^n to bloom,
   'a`nqos flower.] (Bot.)
   The period or state of full expansion in a flower. --Gray.
   [1913 Webster]

WordNet (r) 3.0 (2006):

anthesis
    n 1: the time and process of budding and unfolding of blossoms
         [syn: blossoming, flowering, florescence,
         inflorescence, anthesis, efflorescence]