The Collaborative International Dictionary of English v.0.48:

AEstivation \[AE]s`ti*va"tion\, n.
   1. (Zool.) The state of torpidity induced by the heat and
      dryness of summer, as in certain snails; -- opposed to
      hibernation.
      [1913 Webster]

   2. (Bot.) The arrangement of the petals in a flower bud, as
      to folding, overlapping, etc.; prefloration. --Gray.
      [1913 Webster] [Spelt also estivation.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

aestivation
    n 1: (zoology) cessation or slowing of activity during the
         summer; especially slowing of metabolism in some animals
         during a hot or dry period [syn: estivation,
         aestivation]
    2: (botany) the arrangement of sepals and petals in a flower bud
       before it opens [syn: estivation, aestivation]