The Collaborative International Dictionary of English v.0.48:

Synanthesis \Syn`an*the"sis\ (s[i^]n`[a^]n*th[=e]"s[i^]s), n.
   [NL., fr. Gr. sy`n with + Gr. 'a`nqhsis bloom.] (Bot.)
   The simultaneous maturity of the anthers and stigmas of a
   blossom. --Gray.
   [1913 Webster]