The Collaborative International Dictionary of English v.0.48:

Locus \Lo"cus\, n.; pl. Loci, & Loca. [L., place. Cf.
   Allow, Couch, Lieu, Local.]
   1. A place; a locality.
      [1913 Webster]

   2. (Math.) The line traced by a point which varies its
      position according to some determinate law; the surface
      described by a point or line that moves according to a
      given law.
      [1913 Webster]

   Plane locus, a locus that is a straight line, or a circle.


   Solid locus, a locus that is one of the conic sections.
      [1913 Webster]