The Collaborative International Dictionary of English v.0.48:

Thyrsus \Thyr"sus\, n.; pl. Thyrsi. [L., fr. Gr. ?. Cf.
   Torso.]
   1. A staff entwined with ivy, and surmounted by a pine cone,
      or by a bunch of vine or ivy leaves with grapes or
      berries. It is an attribute of Bacchus, and of the satyrs
      and others engaging in Bacchic rites.
      [1913 Webster]

            A good to grow on graves
            As twist about a thyrsus.             --Mrs.
                                                  Browning.
      [1913 Webster]

            In my hand I bear
            The thyrsus, tipped with fragrant cones of pine.
                                                  --Longfellow.
      [1913 Webster]

   2. (Bot.) A species of inflorescence; a dense panicle, as in
      the lilac and horse-chestnut.
      [1913 Webster]

WordNet (r) 3.0 (2006):

thyrsus
    n 1: a dense flower cluster (as of the lilac or horse chestnut)
         in which the main axis is racemose and the branches are
         cymose [syn: thyrse, thyrsus]