The Collaborative International Dictionary of English v.0.48:

Least \Least\ (l[=e]st), a. [OE. last, lest, AS. l[=ae]sast,
   l[=ae]sest, superl. of l[=ae]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.
   [1913 Webster]

   Note: Least is often used with the, as if a noun.
         [1913 Webster]

               I am the least of the apostles.    --1 Cor. xv.
                                                  9.
         [1913 Webster]

   At least, or At the least, at the least estimate,
      consideration, chance, etc.; being no less than; hence, at
      any rate; at all events; even. See However.
      [1913 Webster]

            He who tempts, though in vain, at least asperses
            The tempted with dishonor.            --Milton.
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            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.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Least \Least\, conj.
   See Lest, conj. [Obs.] --Spenser.
   [1913 Webster] Leastways

The Collaborative International Dictionary of English v.0.48:

Least \Least\, adv.
   In the smallest or lowest degree; in a degree below all
   others; as, to reward those who least deserve it.
   [1913 Webster]

WordNet (r) 3.0 (2006):

least
    adv 1: used to form the superlative; "The garter snake is the
           least dangerous snake" [syn: least, to the lowest
           degree] [ant: most, to the highest degree]
    adj 1: the superlative of `little' that can be used with mass
           nouns and is usually preceded by `the'; a quantifier
           meaning smallest in amount or extent or degree; "didn't
           care the least bit"; "he has the least talent of anyone"
           [ant: most(a)]
    n 1: something that is of no importance; "it is the least I can
         do"; "that is the least of my concerns"

Moby Thesaurus II by Grady Ward, 1.0:

43 Moby Thesaurus words for "least":
   at a disadvantage, at the nadir, below, below the mark, few,
   homely, humble, humble-looking, humble-visaged, humblest,
   in the gutter, inglorious, innocuous, least of all, less, littlest,
   low, lowest, lowliest, lowly, mean, minim, minimum, minority,
   minority group, modest, plain, poor, short of, shortest, simple,
   slightest, small, smallest, teachable, the few, the minority,
   under, under par, undistinguished, unimportant, unpretentious,
   we happy few