Plane locus

from 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]