Source : Webster's Revised Unabridged Dictionary (1913)

Petalism \Pet"al*ism\, n. [Gr. ?, fr. ? a leaf: cf. F.
   p['e]talisme.] (Gr. Antiq.)
   A form of sentence among the ancient Syracusans by which they
   banished for five years a citizen suspected of having
   dangerous influence or ambition. It was similar to the
   ostracism in Athens; but olive leaves were used instead of
   shells for ballots.