The Collaborative International Dictionary of English v.0.48:

Lituus \Lit"u*us\, n.; pl. Litui. [L.]
   1. (Rom. Antig.)
      (a) A curved staff used by the augurs in quartering the
          heavens.
      (b) An instrument of martial music; a kind of trumpet of a
          somewhat curved form and shrill note.
          [1913 Webster]

   2. (Math.) A spiral whose polar equation is r^2[theta] = a;
      that is, a curve the square of whose radius vector varies
      inversely as the angle which the radius vector makes with
      a given line.
      [1913 Webster]