*lituus*

‖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 r2θ = 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 ]