The Collaborative International Dictionary of English v.0.48:

Ambitus \Am"bi*tus\ ([a^]m"b[i^]*t[u^]s), n. [L. See Ambit,
   Ambition.]
   1. The exterior edge or border of a thing, as the border of a
      leaf, or the outline of a bivalve shell.
      [1913 Webster]

   2. (Rom. Antiq.) A canvassing for votes.
      [1913 Webster] ambivalence