Source : Webster's Revised Unabridged Dictionary (1913)

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

   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.

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

   {Solid locus}, a locus that is one of the conic sections.

Source : WordNet®

locus
     n 1: the scene of any event or action (especially the place of a
          meeting) [syn: {venue}, {locale}]
     2: the specific site of a particular gene on its chromosome
     3: the set of all points or lines that satisfy or are
        determined by specific conditions; "the locus of points
        equidistant from a given point is a circle"
     [also: {loci} (pl)]

loci
     See {locus}