The Collaborative International Dictionary of English v.0.48:

Limacon \Li`ma`[,c]on"\ (l[-e]`m[.a]`s[^o]N"), n. [F.
   lima[,c]on, lit., a snail.] (Geom.)
   A curve of the fourth degree, invented by Pascal. Its polar
   equation is r = a cos [theta] + b.
   [1913 Webster]