Source : Webster's Revised Unabridged Dictionary (1913)

Least \Least\, a. [OE. last, lest, AS. l?sast, l?sest, superl.
   of l?ssa less. See {Less}, a.] [Used as the superlative of
   little.]
   Smallest, either in size or degree; shortest; lowest; most
   unimportant; as, the least insect; the least mercy; the least
   space.

   Note: Least is often used with the, as if a noun.

               I am the least of the apostles.    --1 Cor. xv.
                                                  9.

   {At least}, or {At the least}, at the least estimate,
      consideration, chance, etc.; hence, at any rate; at all
      events; even. See {However}.

            He who tempts, though in vain, at least asperses The
            tempted with dishonor.                --Milton.

            Upon the mast they saw a young man, at least if he
            were a man, who sat as on horseback.  --Sir P.
                                                  Sidney.
      

   {In least}, or {In the least}, in the least degree, manner,
      etc. ``He that is unjust in the least is unjust also in
      much.'' --Luke xvi. 10.

   {Least squares} (Math.), a method of deducing from a number
      of carefully made yet slightly discordant observations of
      a phenomenon the most probable values of the unknown
      quantities.

   Note: It takes as its fundamental principle that the most
         probable values are those which make the sum of the
         squares of the residual errors of the observation a
         minimum.

Source : WordNet®

least squares
     n : a method of fitting a curve to data points so as to minimize
         the sum of the squares of the distances of the points
         from the curve [syn: {method of least squares}]